Scrub the clams under cold running water. Discard cracked clams and any open clams that do not close when gently tapped.

Add 1/4 cup water to a large pot, bring to a boil, then add the clams and cover. Steam for about 5–7 minutes, checking as they open. Discard any clams that remain closed after cooking.

Transfer the opened clams to a plate. Strain the steaming liquid through a fine-mesh strainer and reserve 2 tablespoons. Carefully remove and discard the empty top shells, leaving the clam meat in the deeper half shells.

Melt 2 tablespoons butter in a skillet over medium heat. Add the minced shallot and cook for 1 minute, then add the garlic and cook for 30 seconds until fragrant.

Add the chopped cherry tomatoes and cook for 2–3 minutes until softened and some of their moisture evaporates. Stir in 2 tablespoons strained clam liquid, 1 tablespoon lemon juice, black pepper, optional red pepper flakes, and 1 tablespoon chopped tarragon.

Remove the skillet from the heat and stir in the remaining 2 tablespoons butter until glossy. Preheat the broiler to high and set a rack about 6 inches from the heat source.

Nestle the clam half shells securely on a rimmed baking sheet lined with crumpled aluminum foil. Spoon a little tomato-tarragon butter over each clam, distributing the sauce evenly.

Broil for 2–3 minutes, watching closely, until the butter is bubbling and the tomato topping begins to lightly brown. Do not overcook.

Sprinkle with the remaining tablespoon of fresh tarragon and serve immediately with lemon wedges and crusty bread, if desired.