Preheat a skillet with olive oil over medium heat. Sauté the peppers and garlic until softened, 3 to 4 minutes. Remove peppers into a bowl and discard garlic clove. Turn heat to medium high, then brown the sausages on all sides Add sautéed peppers back to the pan, cover, lower heat, and simmer until completely cooked, 8 to 10 minutes. Split and toast sub rolls, then fill with sausage and peppers.