1/4 cup extra-virgin olive oil
2 tablespoons chopped green garlic or 1 regular garlic clove, blanched
1/4 teaspoon fleur de sel or coarse kosher salt
3/4 cup mayonnaise, divided
2 teaspoons fresh lemon juice
2 (3-ounce) packages thinly sliced pancetta (Italian bacon; about 30 slices)
12 (1/2-inch-thick) slices brioche or egg bread, lightly toasted
1 large bunch mizuna or arugula, torn into 2-inch pieces
3 beefsteak tomatoes, cut into 1/4-inch-thick rounds
Blend olive oil, garlic, and 1/4 teaspoon salt 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.
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.