Add butter and chopped onion to med sized saucepan. Cook over med. heat until onions are translucent. Add garlic.
Add flour and stir until well combined. Cook for 3-4 minutes.
Gradually whisk in milk, salt, pepper, and nutmeg. Once well mixed, increase heat and bring to a boil, stirring constantly. Cook (still stirring), until sauce thickens.
Remove from heat and stir in parmesan cheese
For "Mock Lasagna" Ravioli Bake:
Using a large oven proof casserole, spread about 1 1/2 cups sauce on bottom of pan.
Next layer on raviolis. Top with diced chicken & chiles. Top layer of jack & mozzarella cheeses. Top with a layer of parmesan.
Repeat layers: sauce, ravioli, chicken, cheeses, 2 more times.
Finish with a cheese layer.
Bake at 350* for approx 30 minutes covered, then bake for another 15 minutes uncovered until hot, bubbly, and just starting to brown.