Core and seed tomatoes if very watery: halve tomatoes crosswise and gently squeeze out large seeds and excess juice into a bowl; finely dice the tomato flesh to yield about 3 cups. Transfer diced tomatoes to a medium mixing bowl.

Add minced garlic, sliced basil, red wine vinegar, 3 tablespoons extra-virgin olive oil, kosher salt, black pepper, and red pepper flakes (if using) to the bowl with tomatoes.

Stir gently to combine. Taste and adjust seasoning — add up to 1/2 teaspoon more salt or another 1/2 tablespoon vinegar or olive oil as needed to balance acidity and texture. Let the mixture rest at room temperature for at least 10–15 minutes (or up to 1 hour) to allow flavors to meld. If the mixture becomes too watery, spoon off excess liquid or drain for a minute before assembling.

Bread and assembly

Prepare and grill bread

Preheat a grill pan or outdoor grill to medium-high (about 375–425°F / 190–220°C). Brush both sides of each levain slice lightly with the 2 tablespoons extra-virgin olive oil.

Place bread slices on the hot grill or grill pan. Grill 1.5–2.5 minutes per side, until grill marks appear and bread is crisped but not burned. Remove bread and immediately rub the top side of each slice with the cut side of the halved garlic clove to impart a subtle garlic flavor.

If desired, lightly drizzle a little extra-virgin olive oil over the grilled side and sprinkle a pinch of flaky sea salt on each slice.

Assemble bruschetta

Spoon the tomato topping generously onto each grilled bread slice, using a slotted spoon if there is excess liquid. Press slightly so the topping adheres. Garnish with an extra small basil leaf if available and finish with another light pinch of flaky sea salt and a small drizzle of olive oil.

Serve immediately while the bread is still warm and crisp. Leftover topping can be refrigerated in an airtight container for up to 24 hours; do not refrigerate assembled bruschetta (bread will soften).