Preheat oven to 400°F (200°C). Brush squash halves with olive oil, season, and roast cut side down for 40–45 minutes until tender.
Cook chopped bacon in a skillet until crispy. Set aside, reserving 1 tablespoon bacon fat.
In the same skillet, sauté onion until translucent, add garlic and cook briefly, then stir in spinach until wilted.
Add feta, Parmesan, and half the bacon to spinach mixture. Season with salt and pepper.
Flip roasted squash cut side up, mash slightly, and fill with spinach mixture.
Bake 10–15 minutes until warm and slightly golden. Top with remaining bacon and herbs before serving.