Butternut Squash Vegducken

Ingredients:
1 cup pecans
1 (7 1/2"-long) zucchini
1 (9 1/2"-long) globe eggplant
1 (11 1/2"-long) butternut squash
2 scallions
2 garlic cloves, divided
1 shallot, coarsely chopped
1/4 pound shiitake mushrooms, trimmed, coarsely chopped
1 tablespoon olive oil
9 sprigs thyme, divided
1 cup (2 sticks) unsalted butter
1/2 teaspoon red pepper flakes
2 tablespoons pure maple syrup
2 large eggs
1 cup finely grated Parmesan
1/2 cup fine plain breadcrumbs
6 tablespoons coarsely chopped parsley, divided
Kosher salt
2 tablespoons mint, coarsely chopped
1 tablespoon fresh lemon juice
Kitchen twine

Instructions:
Preheat oven to 350°F. Toast pecans on a rimmed baking sheet, tossing once, until fragrant and slightly darkened, 10–15 minutes. Let cool, then coarsely chop and set aside.
Increase oven temperature to 400°F. Line a rimmed baking sheet with aluminum foil.
Cut zucchini in half lengthwise. Using a medium-size quick-release ice cream scoop or heavy spoon, scoop out insides, leaving a small divot down the center. Reserve zucchini filling.
Cut eggplant in half lengthwise and scoop out insides, leaving a 1/4" border on all sides and creating a divot deep enough to fit zucchini halves inside. Reserve eggplant filling.
Cut butternut squash in half lengthwise and remove seeds. Scoop out insides, leaving a 1/2" border on all sides and creating a divot deep enough to fit eggplant halves inside. Reserve squash filling.
Using a fork, pierce insides of squash and zucchini halves. Using a sharp knife, make shallow crosshatch marks inside of eggplant, being careful not to pierce the skin. Trim scallions to match the length of the squash.
Coarsely chop 1 garlic clove. Combine chopped garlic, shallot, mushrooms, zucchini filling, eggplant filling, and squash filling in a large bowl. Working in batches, pulse in a food processor until finely chopped.
Heat oil over medium in a large skillet. Add vegetable purée and 3 thyme sprigs. Cook, stirring occasionally, until mixture begins to brown, about 5 minutes. Return to bowl and let cool.
Meanwhile, smash and peel remaining garlic clove and combine with butter, red pepper flakes, and remaining 6 thyme sprigs in a small pot. Cook over medium heat, stirring occasionally, until butter is melted, then stir in maple syrup.
Pluck out thyme sprigs from vegetable mixture. Stir in eggs, Parmesan, breadcrumbs, 3 Tbsp. parsley, and 1 tsp. salt.
Place squash halves, cut side up, on prepared baking sheet. Brush inside of each with maple syrup butter and season with 1/2 tsp. salt. Using the back of a spoon, press 3/4 cup vegetable mixture into each half until interior is fully coated. Sprinkle with 1/4 cup reserved pecans.
Nestle eggplant halves, cut side up, inside squash halves. Brush inside of each eggplant half with maple syrup butter and season with 1/2 tsp. salt. Using the back of a spoon, press 1/3 cup vegetable mixture into each half until interior is fully coated. Sprinkle with 1/4 cup pecans.
Nestle zucchini halves, cut side up, inside eggplant halves. Brush inside of each zucchini half with maple syrup butter and season with 1/8 tsp. salt. Using a spoon, fill zucchini halves with 1/4 cup vegetable mixture, spreading it flat. Sprinkle with 1/4 cup pecans (reserve remaining pecans), then lay scallions down the middle.
Cut 3 (18") lengths of kitchen twine. Slip twine under one squash half, then top with second squash half, so that cut sides face each other, and press down to seal. Tightly tie twine around squash to secure. Brush top with maple syrup butter (reserve remaining butter) and season with 1/2 tsp. salt. Wrap squash in aluminum foil and place in the center of the baking sheet. Using 2 loaf pans or small metal bowls turned upside down, keep squash secure on baking sheet.
Bake until squash is tender to the touch, 1 hour 45 minutes–2 hours. Remove foil and let rest 20 minutes.
Meanwhile, pluck out thyme from remaining maple syrup butter, heat over medium-low until warm, then stir in mint, lemon juice, 1 tsp. salt, and remaining 3 Tbsp. parsley.
Place vegducken on a cutting board and cut into 1" slices with a serrated knife, transferring to serving plates as you go. Spoon herb butter over slices, garnish with remaining pecans, and serve.