Preparation **For aioli:**Blend olive oil, garlic, and 1/4 teaspoon fleur de sel in processor until garlic is minced. Add 2 tablespoons mayonnaise and blend well. Transfer to small bowl; whisk in remaining mayonnaise and lemon juice. Do ahead: Can be made 1 day ahead. Cover; chill. **For sandwiches:**Preheat oven to 450°F. Arrange pancetta slices in single layer on 2 large rimmed baking sheets. Bake until crisp, about 10 minutes. Transfer to paper towels to drain. Place toast on work surface. Spread with aioli. Divide mizuna among 6 toast slices; top with tomatoes, then pancetta, dividing equally. Top with remaining 6 toast slices, aioli side down. Cut each sandwich in half and serve.
Preparation **For aioli:**Blend olive oil, garlic, and 1/4 teaspoon fleur de sel in processor until garlic is minced. Add 2 tablespoons mayonnaise and blend well. Transfer to small bowl; whisk in remaining mayonnaise and lemon juice. Do ahead: Can be made 1 day ahead. Cover; chill. **For sandwiches:**Preheat oven to 450°F. Arrange pancetta slices in single layer on 2 large rimmed baking sheets. Bake until crisp, about 10 minutes. Transfer to paper towels to drain. Place toast on work surface. Spread with aioli. Divide mizuna among 6 toast slices; top with tomatoes, then pancetta, dividing equally. Top with remaining 6 toast slices, aioli side down. Cut each sandwich in half and serve.