Preheat oven to 375°F
In a large pot over medium high heat, heat olive oil and add ground meat. Cook until no longer pink, breaking up pieces with a wooden spoon as it cooks. Add tomato sauce and mix to combine; set aside.
Bring a large pot of water to a boil, prepare an 9 x 13-inch baking pan with a scoop (about 1/2 -3/4 cup) of tomato sauce along the bottom; set aside
Place one sheet of pasta into the boiling water and cook for 1-2 minutes; remove from pot (making sure to drain all the water) and place on top of the sauce coated baking dish (depending on your pasta sheets, you may have to cut some to fit the bottom of your pan - although it doesn't really have to fit perfectly). Top with a handful of grated cheese and continue layering pasta/sauce/cheese until you have the top (last) pasta sheet placed on as the final layer.
To make the bechamel, melt the butter in a medium saucepan over medium heat. Whisk in flour and continue whisking until it turns into a paste/roux, about 2 min (do not let it brown). Gradually add milk and continue to whisk and continue cooking until sauce thickens, 4 to 5 more min. Season with salt and pepper.
Pour bechamel over the final (last) pasta sheet that has been placed on the top of the lasagna (so the last sheet of pasta will not have anything on it except bechamel) and sprinkle with the last bit of mozzarella, if you have some left and the parmigiano. Bake for about 40 minutes or until warm all the way through.
If you prefer a crispy top, place the lasagna under the broiler for a few minutes to brown the top!