Preheat oven to 350 degrees F.
Cut 1 sheet of 12 Party Rolls in half horizontally, keeping the rolls connected. Arrange both half-sheets of rolls on a 13×9” baking sheet, cut-side-up; brush with olive oil and bake in preheated oven for about 3 minutes or until crisp and lightly toasted.
Slice plant-based sausage links into ¼” thick slices and add to a medium skillet over medium-high heat, or if using ground sausage mixture, add to heated skillet and break up into crumbles. Cook plant-based sausage according to package instructions or until slightly seared and heated through.
Meanwhile, begin layering flatbread: Top toasted roll half-sheets with sliced or shredded cheese and sprinkle with Italian seasoning. Add thinly sliced red onion and artichoke hearts (if using).
Once sausage is finished cooking, transfer to a paper-towel lined plate to drain. Add diced mushrooms to the same skillet (with a bit of olive oil if necessary) and sauté over medium heat until tender.
Top flatbread with cooked sausage and mushrooms. Layer on olives and sprinkle with red pepper flakes.
Bake prepared flatbreads for 10-12 minutes or until cheese has melted.
Remove from oven and top with fresh basil and parmesan cheese, if desired. Cut into squares and serve.