1. Heat 3 tablespoons olive oil in a large heavy skillet over medium-high heat until hot. Add the pepper, onion, garlic, 3/4 teaspoon salt, and 1/2 teaspoon pepper and fry, stirring occasionally until soft and beginning to brown, about 15 minutes.
2. Remove the sausage meat from the casings, then form into 4 (4 1/2-inch) patties. Heat a large heavy skillet or griddle over medium-high heat and add the remaining 1 tablespoon oil. Sear the burgers, turning once, until browned and just cooked through, 10 to 12 minutes.
3. Slice the focaccia pieces in half horizontally to make a top and bottom bun. Put lettuce then the burgers on the bottom portion of the bread, add a mound of pepper-onion mixture and cover with the focaccia top. Serve.