Preheat oven to 425 degrees. Lightly spray a baking sheet with nonstick spray; set aside.
Prepare Tomato Sauce:
Heat 2 teaspoons olive oil to a medium skillet over medium heat.
Add garlic and cook until fragrant, about 30 seconds.
Stir in tomatoes and cook until softened, about 5 minutes.
Mix in balsamic vinegar, then cook until thick.
Remove sauce from heat. Stir in basil and season to taste with salt. Set aside.
Prepare Burgers:
While sauce is cooking, brush portobellos lightly with olive oil.
Roast mushrooms for 10 minutes, then flip and cook for 10 minutes more (or until tender). Remove from oven, then season to taste with salt and pepper.
Once mushrooms start cooking, heat a grill pan over medium heat.
Brush halloumi with remaining olive oil, then add to hot pan.
Cook halloumi until browned grill marks appear, then flip and cook on other side. Remove from heat.
To assemble burgers, top bun with a small handful of lettuce, pickled onions, portobellos, tomato sauce, and cheese.
Serve burgers immediately. Enjoy!
Notes
I prepared this recipe indoors using a grill pan for the cheese and an oven for the mushrooms. However, you could grill both on an outdoor grill over medium heat. If you don't have a grill pan, feel free to fry the cheese in a skillet instead. Make sure to read all the info before this recipe card for more tips + tricks!