Prepare tomatoes: Dice tomatoes into ¼–½" pieces. If very juicy, scoop out some seeds and excess liquid to avoid soggy topping. Place diced tomatoes in a mixing bowl.
Build the topping: Mince 1 garlic clove and add to the tomatoes. Add minced shallot/red onion if using. Stir in 2 tbsp extra-virgin olive oil and the balsamic or red wine vinegar (if using). Add the sliced basil, season with kosher salt and freshly ground black pepper, and gently toss. Let rest 10–20 minutes at room temperature to marry flavors.
Preheat for toasting: Choose your toasting method—broiler (oven), grill pan, or outdoor grill. Preheat accordingly (broiler on high, grill pan medium-high, or grill to medium-high heat).
Toast the bread: Lightly brush both sides of each bread slice with olive oil. Toast 1–2 minutes per side under the broiler or on the grill/grill pan until golden and blistered. Remove and place on a rack or board.
Rub with garlic: While toast is still warm, rub the toasted side with the cut side of the remaining garlic clove—one swipe per slice for a subtle garlic aroma.
Assemble just before serving: Spoon the tomato mixture atop each garlic-rubbed slice (about 1 heaping tablespoon per slice). Drizzle with a little extra-virgin olive oil, scatter a basil leaf, and finish with flaky sea salt and freshly ground pepper. Add shaved Parmesan or a tiny balsamic drizzle if desired.
Serve immediately: Serve on a long board or platter. If serving later, keep tomato mix refrigerated and toasts separate—assemble right before serving to preserve crunch.