Heat oil in a large nonstick skillet over medium-high heat. Add chicken, season with salt and pepper, and cook until no longer pink, about 5 minutes. Reduce heat to medium, add onions, and grate in garlic. Cook and stir until onions are translucent, about 5 minutes.

Stir garam masala, paprika, and a bit more salt into the skillet. Stir in tomatoes and tomato paste. Add a couple handfuls of spinach; cook until wilted, 2 to 3 minutes. Repeat until all spinach is incorporated into the sauce. Cover and simmer until flavors combine, 10 to 15 minutes.