Clean the portobello mushrooms by wiping caps with a damp paper towel. Remove stems and scrape out the dark gills with a spoon if you prefer a milder flavor. Slice each mushroom into 1/4-inch thick slices (you should get about 6–8 slices per mushroom).

Assemble greens and vegetables

In a large bowl, combine the mixed salad greens, halved cherry tomatoes, sliced cucumber, and thinly sliced red onion. Toss gently to mix.

Finish salad

Once mushrooms are cooked and slightly cooled, arrange them on top of the tossed greens. Sprinkle crumbled feta and chopped parsley evenly over the salad.

Marinade / Dressing

Make the dressing: In a small bowl or jar, whisk together 4 tablespoons olive oil, 2 tablespoons red wine vinegar, 1 teaspoon Dijon mustard, minced garlic, 1 tablespoon fresh lemon juice, 1/2 teaspoon honey or maple syrup, 1/2 teaspoon dried oregano, 1 teaspoon salt, and 1/2 teaspoon freshly ground black pepper until emulsified. Taste and adjust salt, pepper, or acidity as desired.

Reserve about 1 tablespoon of the dressing to brush on mushrooms while cooking. Dress the salad greens with the remaining dressing just before serving (about 2–3 tablespoons, or to taste).

For Cooking

Heat a large skillet over medium-high heat. Add 1 tablespoon butter or olive oil. When hot, add the sliced portobello mushrooms in a single layer (work in two batches if needed). Sauté undisturbed for 2–3 minutes so they get a good sear, then toss and cook another 2 minutes until browned and tender.

Optional steaming method: After searing, add 2 tablespoons water to the pan and cover for 1 minute to steam and soften the mushrooms further, then remove cover and allow any liquid to evaporate. Brush the reserved 1 tablespoon dressing over mushrooms during the final minute of cooking.

Transfer cooked mushrooms to a plate and let rest 2 minutes, then place on top of the dressed salad as directed above.