3 cups arugula
3 ripe tomatoes, thinly sliced
3 tablespoons unsalted butter
4 slices country-style bread, each 1/2-inch thick
Pancetta, recipe follows
1/2 pound whole-milk mozzarella cheese, sliced into 12 slices
Sea salt, preferably gray salt
Basil Mayonnaise, recipe follows
1 pound pancetta
Drizzle extra-virgin olive oil
16 large fresh basil leaves
1 1/2 cups mayonnaise
Fresh ground black pepper