Cook spinach:
Cook the spinach according to package directions. Squeeze out excess liquid and reserve. Set the spinach aside.
Start sauce:
Melt butter in a large saucepan over medium heat. Add shallots and cook until soft, about 2 minutes. Stir in the garlic, and cook until fragrant, about 1 minute.
Add the flour and stir until smooth, but not brown, about 30 seconds. Reduce the heat to medium-low.
Cook sauce, add cheese and spinach:
While stirring, slowly add the reserved spinach liquid, milk, and Worcestershire sauce. Stir continuously to avoid any lumps. Cook until smooth and thick, about 2 minutes.
Add the cheese a handful at a time, stirring continuously until it is fully melted.
Fold in the cooked spinach and spinach leaves. Add salt and pepper.
Fill dish:
Transfer to a 1 ½-quart vegetable-sprayed casserole dish.
To serve immediately: Preheat the broiler to high. Combine the oil and breadcrumbs in a small bowl. Top the casserole with the breadcrumbs, and broil until golden, about 3 minutes.
Combine the oil and breadcrumbs in a small bowl. Top the casserole with the breadcrumbs, and broil until golden, about 3 minutes.
To serve the following day: Preheat the oven to 350°F. Place the casserole dish, covered, in the oven, and heat through, 20 to 25 minutes.
Remove from the oven. Combine the oil and breadcrumbs in a small bowl. Top the casserole with the breadcrumbs, and broil until golden, about 3 minutes.