Melt butter in a medium saucepan. Whisk in the flour, and cook until lightly brown. Graduallyadd milk, whisking until smooth. Cook over medium high heatuntil slightly thickened. Remove from heat.
Coat a 9×13 baking pan with non-stick spray. Coat the bottom of the pan with a thin layer of sauce. Cover the sauce with 6 tortillas, overlapping them slightly. Spread half the ricotta mixture over thetortillas, followed by half the chicken, 1 cup of grated cheese, 1/3 of the green onions, and 1 cup of sauce.
Uncover, sprinkle remaining cheese and green onion over the top. Bake, uncoered, another 10minutes, until cheese melts. Remove from oven and let cool for 15 minutes.