Melt the butter over medium-high heat and
add the flour, stirring it to make a roux.
Cook it for a minute or so.
Add the milk and stir until
the sauce is thickened and bubbly.
Add the tuna, peas and cheese and stir.
Turn off the heat but keep the pot on
the element until the peas are hot enough.
If the sauce is too thick, add more milk.
I don't often drain the tuna if the sauce looks a bit thickish.
Serve this over biscuits, English muffins or toast.