Instructions

Adjust oven rack to upper-middle position and heat oven to 400 degrees. Toss 1 pound cherry tomatoes; 1 red onion, cut through root end into ½‐inch-thick wedges; quartered stems of 4 large portobello mushrooms; ¼ cup extra-virgin olive oil; ¾ teaspoon table salt; and ¼ teaspoon pepper together on rimmed baking sheet, then spread into even layer. Brush 4 large portobello mushroom caps, gills removed, all over with remaining 2 tablespoons extra-virgin olive oil, then sprinkle them with ¼ teaspoon salt and ¼ teaspoon pepper. Nestle mushroom caps into tomato mixture on sheet, arranging 1 mushroom cap in each quadrant of sheet. 
Combine 10 ounces frozen chopped spinach, thawed and squeezed dry; 5 ounces crumbled feta cheese; 4 ounces whole-milk ricotta cheese; ½ cup panko bread crumbs; ½ cup toasted walnuts, coarsely chopped; ½ cup grated Parmesan cheese; 3 thinly sliced scallions; ⅓ cup chopped fresh dill; ⅓ cup chopped fresh parsley; 1 lightly beaten large egg; remaining ¼ teaspoon salt; and remaining ½ teaspoon pepper in bowl, stirring until well combined. Divide spinach mixture evenly among mushroom caps, filling cavity to each edge. Transfer sheet to oven and roast for 15 minutes. 
Remove sheet from oven and increase oven temperature to 475 degrees. Sprinkle mushroom caps evenly with remaining ¼ cup grated Parmesan cheese. Return sheet to oven and roast until cheese is golden and cherry tomatoes are blistered and lightly charred, 10 to 15 minutes.
Using large spoon, gently smash about half of cherry tomatoes on sheet to release juices, then stir to combine with roasted onions and any pan juices. Sprinkle with remaining 1 tablespoon chopped fresh dill and remaining 1 tablespoon chopped fresh parsley. Serve.