8 large portobello mushroom caps (about 4 oz. each), gills removed (see Tip)
2 tablespoons avocado oil plus 1 tsp., divided
½ teaspoon ground pepper, divided
½ cup pizza or tomato sauce
2 cups lightly packed baby spinach, chopped
½ cup sun-dried tomatoes (about 8), chopped
1 (14 ounce) can artichoke hearts, rinsed and chopped
½ cup shredded mozzarella cheese
¼ cup crumbled feta cheese
1 tablespoon lemon juice
2 cups lightly packed baby arugula
Preheat oven to 400 degrees F. Line a large baking sheet with foil and set a wire rack on it. Brush tops of portobello caps with 1 Tbsp. oil and place them, undersides-up, on the rack. Roast for 10 minutes. Flip and roast for 5 minutes more.
Remove the portobellos from the oven and carefully flip them back over so that the undersides are up. Season with 1/4 tsp. pepper. Spread 1 Tbsp. sauce inside each cap. Divide spinach, sun-dried tomatoes, artichokes, mozzarella, and feta among the caps. Sprinkle with Italian seasoning. Return the portobellos to the oven and bake until the cheese is melted and starting to brown, 10 to 15 minutes.
Meanwhile, whisk the remaining 1 Tbsp. plus 1 tsp. oil, the remaining 1/8 tsp. pepper, and lemon juice in a medium bowl. Add arugula and toss to coat.