Combine tomatoes, red onion, garlic, salt, and pepper in a large bowl and toss to combine. Let rest for about 15 minutes.
Brush both sides of bread slices with about 2 Tbsp olive oil. Toast in grill pan over medium heat for about 3 minutes (can also use regular pan or toast on grill). Flip the bread and cover the toasted side with half of the parmesan cheese. Let toast for another 2 minutes, until cheese starts to melt. Remove from grill and let cool before cutting into bite-sized cubes.
Pierce sausage with a fork or knife and cook it over medium on grill pan until cooked through. Let cool slightly before slicing.
Add red wine vinegar, remaining 2 Tbsp olive oil, basil, sliced sausage, and bread cubes into tomato mixture and toss to combine.
Serve with remaining parmesan cheese sprinkled over the top.