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Chop any desired aromatics into small pieces and prepare the oysters. Melt four tablespoons of butter in a large pan. Add the 2 tablespoons flour, stirring constantly for 3-4 minutes. Add the onion/shallots, green onions, and/or celery, if using, and cook for 2 minutes. Stir in the oyster liquid and milk slowly, bringing to a simmer for 3-4 minutes. Stir in salt, pepper, and any desired seasonings while the liquid simmers. Add the oysters to the simmering liquid, along with garlic and parsley if using. Cook for 3-4 minutes, or until the oysters curl and remove from heat. Stir in an extra 1-2 tablespoons of butter for a richer, heartier stew. Serve garnished with parsley, sliced green onions, or oyster crackers.
If you're using onions/shallots, celery, or garlic, chop it up ahead of time to make the cook time easier. For the oysters, you want to prepare them now so that you can add them quickly when needed. Pre-shucked oysters are a good call if you can verify they are relatively fresh by asking the fishmonger. Cooked oysters lose much of the "fresh" flavor anyway, so don't feel like you must shuck them yourself. They are sold at the fish counter. Turn the heat to medium and stir the butter around until just melted. Use a large, deep pan-- you'll need something to contain all the liquid for the stew. Do not let the flour simply sit -- use a whisk or wooden spoon to keep it moving, breaking up any clumps as they form. If you have a flat whisk, use this. The flour should start to take on a slightly caramel color as it cooks and darkens. This is the base of a roux. These ingredients, which form the base of so many soups, stews, and sauces, are called "aromatics." As they cook, they will impart a rich, savory sweetness to the stew, but they are not strictly necessary. Stir and cook for 2 minutes. Add the first third or so of liquid, stirring continually as you do. Adding slowly lets the roux incorporate the cold liquid easily, which prevents "breaking." Use your spoon or whisk to break up anything stuck to the bottom of the pan once the liquid hits, then let the heats slow rise until there are bubbles rising to the surface. Sample the liquid as you season it, making sure it tastes to your liking. If you like spicy food, a dash of cayenne, Tobasco, and/or fresh black pepper will make a delicious, but subtle heat. The important ingredients here are the oysters. Simply toss them in, without the shells, and stir them into the liquid as it returns to a simmer. You do not want to overcook the oysters, which will make them tough and unappetizing. Once the edges start to curl up, cut the heat and start serving. If you're not ready to serve just yet, leave the pot on low, but know you should aim to serve shortly. While not strictly necessary, this extra pat of butter adds a rich, creamy sheen to the top of the stew that sticks to the oysters as you eat them, giving them a rich coating and flavor. This stew is best right off the stove, garnished with a light, refreshing bite of greenery or the crunch of a good oyster cracker. Enjoy!