Heat vegetable oil in a large skillet over medium heat; cook and stir the onion in the hot oil until lightly browned, 5 to 8 minutes. Stir the chicken breast chunks, tomato paste, cinnamon stick, ginger, garlic, coriander, turmeric, garam masala, cayenne pepper, and bay leaf in with the chicken; cook and stir until the chicken is opaque and the spices are fragrant, about 10 minutes.
Stir in the spinach; cook and stir until wilted, about 5 minutes. Mix in the yogurt into the mixture until integrated; stir in the skim milk until thoroughly mixed. Cover the skillet, bring to a simmer, and reduce heat to low; allow to simmer until the chicken is no longer pink inside and the sauce has begun to thicken slightly, about 10 minutes.
