Take a medium-sized saucepan and turn a little heat under it. Add butter to it, and let it get melted for some time. Once properly melted, move toward the next step.

Add flour into the melted butter, and stir well until the flour is well mixed in the butter. You must continue stirring it.

Whisking is very necessary at this point. Bring some seasonings and sprinkle them over the flour and butter mixture.

Once you have simmered this mixture a little, you must pour some milk into it.

Whisk the milk in the flour, until the flour is settled in the milk. Continue whisking and blending.

Keep the flame low so that the sauce will start getting thick just how we want it to be.

The sauce for the Tuna Potato Casserole is ready. Then you must use it in the next step to complete the casserole.

Bring the baking pan, and grease the flour using the melted butter. Then bring the potatoes and after slicing the potatoes spread them in the baking pan.

After spreading the potatoes, bring the drained tuna can and make a layer of tuna over the potatoes.

Bring the chopped onion and spread them at the top of the casserole.

Now bring the sauce and cover the whole layered casserole with your cooked thickened sauce.

Once Every part of the casserole is drizzled with the sauce, put the pan in the oven and start baking.

The baking continues for one hour at least and there you go. The amazing sauce-covered potato Tuna Casserole is ready.

Serve after air cooling for a few minutes.