4 tbsp. extra-virgin olive oil, divided
2 medium zucchini, halved lengthwise and sliced into half moons
8 oz. sliced mushrooms
1/2 small yellow onion, finely chopped
3 cloves garlic, finely chopped
1 tsp. smoked paprika
Pinch saffron threads (optional)
1 (14-ounce) can chopped tomatoes
1 1/2 c. short-grain paella rice (such as Bomba rice)
1 c. fresh or frozen peas
1 c. piquillo peppers, sliced (or roasted red peppers)
5 oz. baby spinach
3 1/2 c. vegetable broth
Chopped fresh parsley, for serving
Lemon wedges, for serving
Preheat oven to 425°F.
In a deep 12-inch cast-iron or other ovenproof skillet, heat 2 tablespoons oil over medium heat. Add zucchini and mushrooms and season with salt. Cook, undisturbed, until golden on one side, about 3 minutes. Toss and cook, sitrring occasionally, until golden and tender, 8 to 10 minutes; transfer to a small bowl.
To same skillet, add 2 tablespoons olive oil and return to medium heat. Add onion and season with salt. Cook, stirring often, until softened, 3 to 5 minutes. Add garlic, smoked paprika, and saffron and cook, stirring, until fragrant, 30 seconds. Add tomatoes and rice and stir to combine. Stir in zucchini mixture, peas, piquillo peppers, and spinach, stirring to wilt spinach. Add broth and ¾ teaspoon salt. Bring to a boil then carefully transfer to the oven. Bake, uncovered, until liquid is absorbed and rice is tender, 20 to 22 minutes.
Transfer skillet to stovetop over medium heat. Cook until bottom of rice is crisp, 2 to 3 minutes. Let rest 5 minutes. Top with parsley and serve with lemon wedges.