1. Cook pasta as directed on package.
2. Drain tuna reserving the liquid, add lemon juice and refrigerate.
3. Meanwhile, melt the 2 tablespoons butter in large skillet over medium heat and saute the onion and mushroom slices until soft.
4. Stir in the flour until blended and cook over low heat until bubbly (stirring constantly).
5. Stir in the evaporated milk and add the reserved tuna oil and cook stirring constantly, until mixture thickens.
6. Stir in salt and pepper and adjust seasoning as needed.
7. Gently stir the tuna with the sauce and heat through.
8. Drained pasta and serve warm with the tuna and sauce mixture.
9. NOTE: Eileen said that she served it with rice instead of the pasta.