Step 1

Preheat oven to 500°. Toss tomatoes with oil on a rimmed baking sheet to coat. Roast until skin is blackened and tomatoes are very soft, 20–25 minutes. Pass tomatoes through finest disk of a food mill or a coarse-mesh sieve into a small saucepan; season with salt. Bring to a simmer over medium-low heat, stirring often, and cook until you have about ½ cup tomato purée. Let cool.

Step 2

Meanwhile, toast fenugreek, cumin, and peppercorns in a dry small skillet over medium heat, tossing often, until fragrant. Let cool and finely grind in spice mill or with mortar and pestle.

Step 3

Mix tomato purée, spice mixture, and mayonnaise in a bowl; season with salt.

Step 4

Do Ahead: Tomato mayonnaise can be made 5 days ahead. Cover and chill.

Step 5

Prepare a grill for medium-high heat. (Alternatively, heat a grill pan over medium-high.) Brush both sides of bread with 2 Tbsp. oil total and grill until lightly charred, about 2 minutes per side.

Step 6

Toss fennel, dill, parsley, lemon juice, and remaining 1 Tbsp. oil in a medium bowl; season with salt. Spread tomato mayonnaise over toast and top each with a few sardines and some fennel salad.