Step 1

Preheat your oven to 350 F. Melt the butter in a 9x13-inch casserole dish while the oven heats and you mix your batter.

Step 2

Once the butter has melted, remove the pan. You may need to use a pastry brush to ensure the butter is evenly spread over the dish.

Step 3

While the oven is preheating, whisk together the eggs in a large mixing bowl. Add the flour, milk, salt, and baking powder. Stir to combine.

Step 4

Add the onion, cheese, and thawed spinach to the bowl. Stir to combine, then pour into your prepared casserole dish.

Step 5

Bake for 30-35 minutes until golden brown on top. Remove and allow to cool 30-40 minutes to set. At this time, bars can either be cut, or refrigerated up to 3 days, reheat before serving (see step 8).

Step 6

Cut into squares and serve warm or at room temperature.

Step 7

If preparing in advance to freeze: Cool the squares, cut, and freeze the spinach squares in a single layer on a baking sheet until solid. Then place in an airtight container. When you wish to serve, place the desired number of squares on a baking sheet and bake in a 325 F oven for 8-12 minutes until warmed through.

Step 8

To reheat refrigerated squares, bake in a 325 F oven for 5-10 minutes until just warmed through. You can also microwave for a few seconds, but the texture is better when warmed in the oven - almost identical to fresh!