Portobello mushrooms are so meaty that even carnivores will be satisfied with this lusty vegetarian dish. Instead of pounding the caps, we weigh them down in the skillet to flatten them while they cook.
- Heat 1 tablespoon oil in a medium nonstick skillet over medium heat. Add onion and cook, stirring occasionally, until brown, 8 to 10 minutes. Reduce heat to very low, season with 1/4 teaspoon pepper and 1/8 teaspoon salt. Continue cooking, stirring occasionally, until caramelized, about 15 minutes. Transfer to a bowl and keep warm.
- Meanwhile, place milk in a small bowl and place breadcrumbs on a large plate. Dip each mushroom cap in milk, then dredge in the breadcrumbs.
- Heat 1 tablespoon oil in a large nonstick skillet over medium heat. Add mushrooms, gill-side down. Place a heavy, heatproof plate or pie pan on top of the mushrooms and cook until golden brown, pressing down on the plate periodically to flatten them, about 6 minutes. Carefully remove the plate using an oven mitt or tongs, add 1 tablespoon oil to the pan and turn the mushrooms over. Replace the plate and cook, pressing the plate once or twice, until the mushrooms are golden brown and cooked through, 5 to 6 minutes more. Remove from heat; cover to keep warm.
- Heat the remaining 1 tablespoon oil in a Dutch oven over medium-high heat. Add garlic and cook until fragrant, 20 to 30 seconds. Add spinach and cook, stirring until just wilted, about 2 minutes. Stir in beans, broth and the remaining 1/4 teaspoon pepper and 1/8 teaspoon salt. Cook, stirring occasionally, until heated through, 1 to 2 minutes.
- Cut each mushroom into thin slices and serve over the spinach. Top with the reserved onions and cheese.