Preheat the oven to 350°F (180°C).
Sauté the onion in olive oil over medium heat for about 8 minutes until soft. Add scallions and cook 2 more minutes.
Add spinach in batches, seasoning each batch with salt and pepper, and cook until wilted and most of the liquid has evaporated. A few tablespoons of liquid remaining is fine.
Remove from heat. Stir in crumbled feta and roasted red pepper (if using). Taste and adjust seasoning. Let cool.
Prepare the pan: Place the tart or pie pan on a baking tray. Brush the inside with melted butter. Sprinkle breadcrumbs evenly across the bottom.
Layer the phyllo: Working quickly, layer the phyllo sheets into the pan, drizzling butter between each layer. Allow excess to hang over the sides.
Make the custard: In a bowl, whisk eggs with half and half, nutmeg, salt, and pepper.
Assemble the quiche: Add the spinach filling to the pan, sprinkle with shredded cheese, and pour the custard over the top.
Fold in the phyllo edges to form a rustic crust. Brush with remaining butter.
Bake for 45 minutes on the center rack until golden and set.
Cool for at least 20 minutes before removing from the tart pan.
