Prepare the Mushrooms:
Clean the mushrooms by wiping them gently with a damp paper towel to remove any dirt. If using larger mushrooms like portobello, you can remove the stems and slice them if desired. For smaller mushrooms like button or cremini, leave them whole.

Make the Garlic Butter:
In a small bowl, melt the butter in the microwave or on the stovetop. Once melted, stir in the minced garlic, olive oil, salt, pepper, thyme, and red pepper flakes (if using). Mix well to combine.

Marinate the Mushrooms:
Place the mushrooms in a shallow dish or a large bowl. Pour the garlic butter mixture over the mushrooms and toss to coat them evenly. Allow the mushrooms to marinate for about 15-20 minutes to absorb the flavors.

Preheat the Grill:
Preheat your grill to medium-high heat (about 375°F to 400°F / 190°C to 200°C).

Grill the Mushrooms:
If you’re using smaller mushrooms, you can place them directly on the grill, using a grill basket to keep them from falling through the grates. For larger mushrooms, you can grill them directly on the grates or use skewers. Grill the mushrooms for about 3-4 minutes per side, or until they are tender and have nice grill marks.

Serve:
Once the mushrooms are grilled, remove them from the heat and place them on a serving plate. Garnish with chopped fresh parsley if desired, and serve hot.