Step 1

Preheat oven to 350°. Remove fronds from fennel bulbs and thinly slice; set aside. Thinly slice fennel bulb; set aside. Thinly slice all but 1 garlic clove. Combine sliced garlic, fennel bulb, onion, anchovies, drained tomatoes, wine, fennel seeds, 1½ tsp. red pepper flakes, and ⅓ cup oil in a large Dutch oven or other heavy pot. Season with a generous pinch of salt and cover pot, leaving lid slightly askew. Bake until tomatoes are dark red and jammy, 1¼–1½ hours.

Step 2

While tomato sauce is cooking away in the oven, mix mayonnaise and lemon juice in a small bowl. Finely grate remaining garlic clove into mayonnaise, season with salt and pepper, and mix together. Cover and set aside.

Step 3

Remove pot from oven and crush tomatoes against the side of the pot with a wooden spoon to break them up. Stir in clam juice and reserved tomato juices; season with salt. Place pot over medium heat and bring to a simmer. Add clams, pushing into the liquid so they are all submerged. Cover pot and cook just until clams start to open, about 5 minutes. Add mussels, cover, and cook just until mussels start to open, about 5 minutes. Remove lid and use spoon to push shellfish into center of pot. Arrange scallops around perimeter, nestling into liquid so they’re just barely submerged. Cover pot again and simmer just until scallops are cooked through, about 4 minutes.

Step 4

Ladle stew into bowls; discarding any clams or mussels that are not open. Drizzle with more oil and sprinkle with more red pepper flakes. Scatter reserved fennel fronds over top and tuck a lemon wedge into each bowl. Slather seasoned mayonnaise over toast and serve alongside.

Step 5

Do Ahead: Tomatoes can be roasted 1 day ahead. Add clam and tomato juices and let cool. Cover and chill. Bring to a simmer over low heat before making the stew.