- 1/2 cup mayonnaise
- 2 teaspoons Dijon mustard
- 1 teaspoon fresh lemon juice
- Kosher salt
- 1 large shallot, thinly sliced into rings
- 1 garlic clove, finely grated
- 3 tablespoons red wine vinegar
- 1/2 baguette, split in half lengthwise
- 4 tablespoons olive oil, divided
- 3 cups cherry tomatoes
- Freshly ground black pepper
- 1 cup (packed) torn basil leaves
- 1/2 cup (loosely packed) parsley leaves with tender stems
- 1/2 teaspoon finely grated lemon zest
- 4 ounces fresh mozzarella, torn into pieces
- Crushed red pepper flakes (for serving)
- Mix mayonnaise, mustard, and lemon juice in a small bowl; season with salt.
- Toss shallot, garlic, and vinegar in another small bowl; season with salt. Let sit 10 minutes. Drain, reserving shallots and vinegar separately in small bowls.
- Prepare a grill for medium-high heat. Brush cut sides of bread with 2 Tbsp. oil total and grill until lightly charred, about 2 minutes per side. (Or broil bread halves, cut side up, until lightly charred, about 2 minutes.) Season with salt; let cool.
- Heat a large cast-iron skillet over medium-high (you can do this directly on the grill grates or on a burner). Toss tomatoes with remaining 2 Tbsp. oil and cook, shaking pan often, until tomatoes are blistered and lightly charred, about 3 minutes. Add tomatoes to bowl with reserved vinegar; season with salt and pepper. Let cool slightly, then add basil, parsley, and lemon zest and toss gently to combine. Taste and season with salt and pepper as needed.
- Spread both halves of baguette with mayonnaise mixture. Top with mozzarella, tomato salad, and shallots, and sprinkle with red pepper flakes.
- Mayonnaise can be made 3 days ahead. Cover and chill.