Melt the butter, add the flour and then stir. Stir constantly for about 1 minute until all flour has mixed in with the melted butter. Then slowly add the milk, stirring constantly. Cook for 2 more minutes until the milk is totally incorporated and the mixture becomes thick and bubbly.

Take pan off the heat and stir in 1 3/4 cup of cheese, reserving the last 1/4 cup to sprinkle on top. If you're using Dijon mustard, add that teaspoon now. Stir until the cheese has melted.

Turn the oven on to 350 degrees F.

Using a paper towel, lightly grease the bottom of your pan with a little butter.

Layer half of your Lima beans in the pan, then pour half of your cheese sauce on top. The melted cheese sauce should fill every nook and cranny.

Repeat with the second layer, nestling in the onion slices and then add a sprinkle of salt and pepper.

Pour the remaining cheese sauce over the second layer of Lima beans, add salt and pepper and then the last 1/4 cup of cheese.

Bake for 30 minutes uncovered. Remove the cover and bake for an additional 25 minutes. You'll know it's done when the Lima beans are soft and the cheese is golden and bubbly.

Allow to sit for 10 minutes before serving .