Wash and dry the heirloom tomatoes. Slice larger tomatoes into 1/2-inch thick wedges; slice smaller ones in half or quarters so pieces are bite-sized. Arrange tomatoes on a large shallow serving platter or bowl in a single layer with some overlap for visual appeal.

Add onion and basil

Scatter the thinly sliced red onion evenly over the tomatoes. Tear the basil leaves by hand into medium pieces (discard stems) and distribute them over and around the tomatoes to preserve bright color and fragrance.

Season

Lightly sprinkle flaky sea salt and freshly ground black pepper over the assembled tomatoes and basil. Hold off on the finishing olive oil until after the vinaigrette is added.

Onion, Serrano & Sherry Vinaigrette

Combine aromatics

In a small bowl or mason jar combine the finely minced red onion and minced serrano chili. If you prefer less heat, ensure the serrano is seeded before mincing. Let sit 2–3 minutes to soften and meld flavors.

Emulsify vinaigrette

Add the sherry vinegar, Dijon mustard, and honey to the aromatics. Whisk briefly to combine. Slowly drizzle in the 6 tablespoons of extra-virgin olive oil while whisking continuously until the vinaigrette is emulsified and slightly thickened. Alternatively, add all ingredients to a jar, close, and shake vigorously for 20–30 seconds.

Season vinaigrette

Taste and season the vinaigrette with 1/4 teaspoon flaky sea salt and 1/4 teaspoon freshly ground black pepper, adjusting to preference. If the dressing tastes too sharp, add another 1/4 teaspoon honey; if too mild, add 1 teaspoon more sherry vinegar.

Marinate onion (optional)

If time allows, let the vinaigrette sit with the minced onion and serrano for 10–15 minutes to mellow the raw onion flavor and infuse heat into the dressing.

Dress the salad

Drizzle about 2–3 tablespoons of the vinaigrette evenly over the tomatoes, basil, and onions on the platter, using more or less to taste. Toss gently with two spoons or lift and turn pieces to coat without bruising the tomatoes. Reserve any leftover vinaigrette for serving.

Finish and serve

Finish with the 1 tablespoon extra-virgin olive oil drizzled over the top for sheen and richness, then taste and add a final pinch of flaky sea salt or grind of black pepper if needed. Serve immediately at room temperature.