Charred Caprese Sandwich

Name: Charred Caprese Sandwich
Ingredients: 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)