Preheat oven to 450˚F. Cut 2 chicken breasts in half to make 4 pieces. Butterfly all 4 pieces & pound out any thick parts to even out chicken (or lightly pound out chicken to ¼-½ inch). This can be done with chicken between to pieces of wax paper or plastic wrap.
In a non-stick skillet, spray generously with non-stick pan spray & saute onion & garlic with a little salt & pepper for a couple minutes until fragrant & onion is slightly translucent. Add your spinach & toss for 30 seconds or until spinach breaks down in size & is "wet." Remove from heat & set aside.
Season both sides of each chicken breasts with salt & pepper. Add ricotta cheese to slightly cooled spinach mixture & spread mixture over the top of the chicken; roll chicken up. Spread pesto sauce on a plate. Roll filled chicken breasts in the pesto sauce & place seam side down on a greased baking sheet.
Bake at 450˚F for 20 minutes (no longer pink.) Remove from oven & top with sauce & mozzarella slices. Return to oven for 3 more minutes or until cheese melts.