Preheat your oven to 425 degrees F. Arrange a wire baking rack inside a rimmed baking sheet (lined with foil, if desired).
Wash the portobello mushrooms off and dry with clean paper towels. Use a paring knife to slice the stems off, and use your hands to pull off any soft parts of the mushroom along the edge that overlaps the gills. Reserve these parts.
Place the mushrooms stem side down on the rack in the baking sheet. Brush with 1/2 tablespoon of olive oil and season with salt and pepper. Turn them stem side up and repeat on the other side with the remaining 1/2 tablespoon olive oil. Roast in preheated oven for 20 minutes.
While the mushrooms are roasting, chop up the stems and any other scraps from the mushrooms. Sauté the onion and chopped mushroom in 1 tablespoon of the olive oil in a medium skillet over medium-high heat with a pinch of salt and pepper until softened and starting to brown (about 3 minutes). Add the baby spinach and sauté until wilted (about 1 minute). Add the chopped garlic and sauté until fragrant (about 1 minute). Add the marinara sauce (1 cup) and stir together. Turn off heat.
In a small bowl, mix together the breadcrumbs (1/4 cup), remaining 1 tablespoon of olive oil, dried oregano (1 teaspoon), and a pinch of salt and pepper.
Remove mushrooms from oven. You may notice pools of liquid inside each cap. Use a paper towel held by tongs to press down in each cap to absorb as much liquid as you can.
Divide the marinara mixture evenly into each mushroom cap, followed by 1/4 cup of mozzarella cheese on each. Sprinkle each evenly with the breadcrumb mixture.
Continue baking at 425 degrees F for 10-15 more minutes, until cheese is melted and breadcrumbs are toasted. Serve hot.
For a lower carb version, omit the breadcrumbs and top with fresh grated parmesan cheese and sprinkle with oregano before baking. Or, top with pepperoni or a low carb alternative, like crumbled pork rinds.
For a gluten-free version, use gluten-free breadcrumbs.
For a vegan version, use your favorite vegan cheese substitute for the mozzarella.
Make it ahead: bake the mushrooms and assemble them as directed and store in the refrigerator for up to three days. Bake for a few more minutes straight from the fridge so they warm through the whole way.