Heat the olive oil in a small pot over medium heat.
Add the fennel seeds, Aleppo-style pepper, and anchovies, swirling and toasting until the spices are fragrant and the anchovies are melted, 2 to 4 minutes.
Remove from the heat and set aside.
Scatter the tomatoes on a large serving platter or in a shallow bowl.
Drizzle with the vinegar and, if you want, scatter with more anchovies (or just serve alongside in their little tin for snacking on).
Spoon the toasted fennel–anchovy mixture over and top with flaky salt and a few grinds of black pepper.
DO AHEAD: The toasted fennel–anchovy oil can be made up to 2 weeks ahead, tightly covered, and stored in the refrigerator.
The tomatoes can be sliced 1 hour ahead, if you must, but are really best if sliced, dressed, and served immediately.
