Step 1

Make the escabeche: In a small skillet over medium heat, warm 1 teaspoon of the oil. Add the onion and cook, stirring, until it begins to soften, about 2 minutes. Add the garlic and stir until fragrant, about 30 seconds. Increase the heat to medium-high and add the vinegar and sugar. Bring to a boil, stirring to dissolve the sugar, and cook until the liquid has reduced by about half, about 6 minutes. Remove from the heat and transfer to a small bowl. Let cool for about 10 minutes.

Step 2

Add the remaining 2 tablespoons of oil to the reduction and stir to combine. Add the mussels and fennel and toss to coat. Cover and refrigerate for at least 2 hours and up to overnight.

Step 3

When ready to serve, arrange the lettuce leaves on a platter so they are like small cups. Divide the escabeche with the marinade among the leaves, top each with 4 pieces of tomato and serve.