Preheat your grill to medium-high heat.
Brush the mushrooms on both sides with the olive oil and season with salt and pepper.
Grill the mushrooms for 4-5 minutes per side, or until they are tender and cooked through.
Remove the mushrooms from the grill and let them cool slightly before slicing them into strips.
Place the bell peppers, onion, feta cheese, and spinach in a large bowl.
Add the sliced mushrooms and toss to combine.
To make the vinaigrette, whisk together the olive oil, vinegar, garlic, mustard, honey, oregano, thyme, salt, and pepper in a small bowl.
Drizzle the vinaigrette over the salad and toss to combine.
Serve the salad immediately. Enjoy!