These grilled stuffed mushroom sliders are a delicious vegetarian option for your next barbecue. Portobello mushrooms are marinated in a flavorful mixture of balsamic vinegar, garlic, and herbs, then stuffed with a mixture of spinach, feta cheese, and breadcrumbs. The mushrooms are grilled until tender and juicy, then served on slider buns with a tangy aioli sauce. These sliders are packed with flavor and make a great appetizer or main dish for any gathering.