Prep Caps → Gently twist stems off portobellos; scrape dark gills with spoon—score tops in crosshatch for marinade magic.

Whisk Marinade → Combine balsamic, soy, olive oil, garlic, paprika in zip bag—massage to blend flavors.

Marinate Mushrooms → Add caps to bag; gently coat and press out air—rest 5 minutes (or up to 1 hour in fridge).

Heat Grill → Preheat to medium-high (400°F); oil grates generously with soaked paper towel.

Grill First Side → Place caps gill-side up; cook 4 minutes until deep char marks form and juices pool.

Flip & Cheese → Turn caps; top each with provolone slice—grill 3-4 minutes until cheese melts and bottoms crisp.

Toast Buns → Split buns on upper rack last 2 minutes—watch closely for golden crunch.

Rest briefly → Transfer mushrooms to plate; rest 3 minutes for juices to redistribute.

Build Burgers → Spread garlic aioli on both bun halves; layer arugula, mushroom, optional toppings.

Serve Immediately → Crown with bun top; dig into dripping, smoky perfection while hot.