Preheat oven to 450 degrees F (230 degrees C). Line a baking sheet with parchment paper.
Arrange tomato slices on the baking sheet and top with about 3 tablespoons olive oil, sea salt, and pepper.
Roast in the preheated oven until tomatoes are wilted, wrinkly, and scorched in some places, 25 to 30 minutes. Set aside to cool.
Heat butter in a large skillet over medium heat. Place bread slices in the skillet and cook until crisp and golden in parts, 2 to 3 minutes per side.
Remove bread from skillet and rub garlic onto each slice; sprinkle sea salt over bread.
Layer mozzarella cheese on the toasted bread and top with avocado slices. Sprinkle sea salt over avocado and squeeze lemon over it. Top with roasted tomatoes and dill. Drizzle remaining 1 tablespoon olive oil over crostini and sprinkle with flaked salt.