Prep the Mushrooms: Slice the Portabella mushrooms into thick 1-inch slices and set aside.
Make the Marinade: In a bowl, combine olive oil, lemon juice, minced garlic, fresh thyme, salt, pepper, and lemon zest. Mix well.
Marinate the Mushrooms: Place the mushroom slices in a shallow dish or zip-top bag and pour the marinade over them. Refrigerate for at least 30 minutes to allow flavors to infuse.
Skewer the Mushrooms: After marinating, thread the mushroom slices onto skewers.
Grill the Skewers: Preheat your grill to medium heat. Grill the skewers for 10-12 minutes, turning occasionally, until mushrooms are tender and lightly charred.
Serve: Remove from the grill, garnish with extra fresh thyme or a squeeze of lemon, and serve hot.