3 bunches spinach (about 3 pounds total), trimmed and washed
Preheat oven to 400 degrees. Place mushrooms, stem side down, on a rimmed baking sheet. In a small bowl, whisk together 2 tablespoons each lemon juice and oil. Brush mushrooms with oil mixture; season with salt and pepper. Roast mushrooms until tender and beginning to release their juices, 15 minutes. Flip and drain juices from sheet. Reserve 2 mushrooms for tomorrow's lunch. Increase oven to 450 degrees.
Meanwhile, in a medium saucepan, heat 1 teaspoon oil over medium-high. Add half the garlic and cook, stirring, until fragrant, 30 seconds. Add beans, 1 teaspoon thyme, and 1/4 cup water. Season with salt and pepper. Bring to a simmer, stirring occasionally, until liquid is almost evaporated, 3 minutes.
In a food processor, pulse bread until coarse crumbs form. Add 1 teaspoon oil and 1 teaspoon thyme; pulse to combine. Top each mushroom with 1/3 cup bean mixture. Reserve remaining beans for lunch. Divide cheese and breadcrumbs among each mushroom. Return sheet to oven and cook until breadcrumbs are golden brown, 5 minutes.
In a large skillet, heat 1 teaspoon oil over medium-high. Add remaining garlic and cook, stirring, until fragrant, 30 seconds. Gradually add half the spinach (reserve remaining spinach for lunch), season with salt and pepper, and toss until wilted. Add lemon zest and 1 teaspoon lemon juice and toss to combine. Serve spinach alongside stuffed mushrooms.
Ready for tomorrow's lunch: Set aside two cooked mushrooms and leftover beans; refrigerate in airtight containers. Wrap cleaned spinach in damp paper towels and refrigerate in a plastic bag.