Cut chicken breasts into cubes. You may wish to pound them thinner if they are very thick.
Heat oil in frying pan. Saute, onions until softened. Add garlic for one minute. Do not burn.
Saute chicken, on both sides. You may need to do this in two sessions, so as not to crowd.
Remove chicken and set aside.
Add can of tomatoes to the pan, reserving some of the juice for a different use.
Reduce tomatoes down to an almost thick tomato puree. This will take about twenty minutes.
In the meantime, cook pasta according to directions, minus a few minutes, as pasta will continue to bake in oven.
Once pasta is complete, drain and toss with grated pecorino romano.
Put into a large baking dish, toss with tomatoes, spinach and half of the mozzarella, keeping enough to top the dish.
Spoon ricotta cheese just here and there.
Top with mozzarella.
Bake at 375 degrees F for 20 minutes or until all the cheese is melted on top.