3 Tbsp unsalted butter
1 large yellow onion, finely diced
1 clove garlic, minced
1/4 cup Flourish Flour
2 1/2 cups milk
2 tsp kosher salt
1/8 tsp nutmeg
5 10 oz boxes spinach, chopped, defrosted
1/2 cup Parmesan cheese, grated
1/2 tsp pepper
1/2 cup Monterey Jack cheese, or Gruyere cheese, shredded
Preheat oven to 400 degrees, lightly coat a 2-qt casserole dish with cooking spray and set aside. Place a large skillet over medium heat then melt butter. Add onions and cook 3–4 minutes until translucent. Add garlic then cook 1 additional minute.
Add flour then cook, stirring constantly for 1 additional minute. Add milk, salt and nutmeg then stir until combined. Bring liquid to a bubble then reduce heat to low and simmer for 10 minutes, stirring frequently to prevent scorching.
Squeeze as much liquid as possible from the spinach—the successful outcome of the dish depends on removing as much water as possible! Add spinach and Parmesan cheese to sauce and stir until completely combined. Taste and season with additional salt and pepper if necessary.
Transfer to prepared baking dish and sprinkle with Monterey Jack or Gruyere cheese. Bake for 28–30 minutes or until cheese becomes golden and dish is hot and bubbly.