In a large saute pan, over medium-high heat add and melt 3 T. butter. Once melted, add sliced leeks.
Saute for about 15 minutes or until tender enough to your liking.
While the leeks are cooking, make the sauce.
In a small saucepan melt the butter over medium-high heat.
Add the flour and cook for about 1 minute.
Whisk in the milk and continue to whisk until the sauce thickens and comes to a boil, about 2 to 3 minutes.
Reduce to a simmer and season with the salt and pepper. Simmer for 2 to 3 more minutes.
Stir in the Gruyere until melted. Keep warm.
Pour cheese sauce over tender leeks and toss to combine. Heat through and serve immediately. Say Grace and enjoy!!