Summer Vegetable Terrine

Ingredients:
"2 1/2 lb mixed baby (1-inch) or medium (2-inch) Chioggia and golden beets with greens (about 16 baby or 6 medium; see cooks note, below)", 2 large leeks, trimmed of any discolored leaves or ends, leaving green parts attached
2 medium carrots, thinly sliced crosswise
2 celery ribs, sliced, reserving leaves
2 shallots, thinly sliced crosswise
2 cups dry white wine
1 teaspoon salt
10 black peppercorns
3 1/4 cups cold water
1/2 cup chopped mixed tender fresh herbs, stems reserved for stock
1 (1/2-oz) bunch fresh chives, coarser bottom third cut off and reserved and remainder chopped
4 1/2 teaspoons gelatin (from two 1/4-oz envelopes)
1/3 lb haricots verts, trimmed
1/3 lb wax beans, trimmed
1/4 cup extra-virgin olive oil, plus additional for oiling terrine
Fleur de sel for sprinkling
Coarsely ground black pepper for sprinkling
Garnish: fresh herb sprigs
"heavy-duty foil; a nonreactive (6-cup capacity) rectangular terrine or loaf pan (see cooks note
below)"

Instructions:
Put oven rack in middle position and preheat oven to 450°F.
Trim beets, leaving 1/2 inch of stems intact. Divide between 2 sheets of heavy-duty foil and wrap foil to enclose beets. Roast in a shallow baking pan until very tender, 1 to 1 1/2 hours. Let steam in foil 15 minutes, then peel beets and cut into 1-inch-wide wedges if large. Season with salt and pepper.
Halve leeks lengthwise, then coarsely chop. Wash leeks in a bowl of water, agitating them, then lift out with a slotted spoon and transfer to a 3-quart saucepan. Add carrots to leeks along with sliced celery, shallots, wine, salt, peppercorns, and 3 cups cold water and bring to a boil, then reduce heat and simmer, uncovered, 30 minutes.
Add celery leaves, herb stems, and coarse parts of chives and simmer 10 minutes. Pour stock through a fine-mesh sieve into a 1-quart glass measure or heatproof bowl, discarding solids. If stock measures more than 2 1/2 cups, return to saucepan and boil until reduced to 2 1/2 cups, about 10 to 15 minutes. If there is less, add water. Season with salt and pepper.
Stir gelatin into remaining 1/4 cup cold water and let stand 1 minute to soften, then add to hot stock, stirring until dissolved. Set aside.
Boil haricots verts and wax beans in separate batches in a 5- to 6-quart pot of boiling salted water, uncovered, until very tender, 6 to 7 minutes per batch, transferring with a slotted spoon to a bowl of ice and cold water to stop cooking. Drain in a colander and pat dry. Transfer to a large bowl and season with salt and pepper.
Very lightly oil terrine, then line long sides and bottom with a sheet of plastic wrap, smoothing out any wrinkles and allowing at least 2 inches of overhang on each side. Pour about 1/2 cup gelatin mixture into terrine and quick-chill in freezer until just set, about 10 minutes.
Lay one third of beans lengthwise over set gelatin layer. Sprinkle with one third of chopped herbs (including chives), then loosely top with half of beets, leaving some space between them (for gelatin to fill and hold vegetables together). Repeat layering with half of remaining beans and herbs and all of beets, then end with a third layer of beans and herbs. Stir remaining gelatin mixture again, then slowly pour in all but 1/2 cup (reserve remainder at room temperature), pushing down vegetables if necessary to just cover with gelatin mixture. Chill, uncovered, until top is set, 1 1/2 to 2 hours.
If reserved 1/2 cup gelatin mixture has begun to set, heat until just liquefied but not hot, then pour over set terrine. Chill until firm, about 2 hours.
Run a thin knife along short sides (ends) of terrine, then invert terrine onto a cutting board, gently pulling on plastic overhang to help unmold (discard plastic wrap). Carefully cut terrine with a very sharp knife into 8 slices, transferring each slice to a plate as cut, using a metal spatula to hold outside of each slice steady. Drizzle oil (1/4 cup) around plates and sprinkle fleur de sel and pepper over oil and terrine.