Season chicken and set aside.
Heat olive oil in a large nonstick skillet.
Add chicken and brown on all sides, about 2 to 3 minutes per side.
Remove chicken from skillet and set aside.
Melt butter in skillet; add diced onion and cook over medium-high heat for 2 minutes; stir in garlic and continue to cook for about 30 seconds, or until fragrant.
Stir in orzo pasta; cook for 2 minutes, stirring frequently.
Add chicken broth and stir to combine.
Arrange prepared chicken over the orzo.
Add tomatoes and spinach on top; season with salt and pepper, and bring mixture to a boil.
Lower the heat to a simmer; cover and cook until the orzo has completely absorbed the liquid and the chicken is cooked through, about 12 to 15 minutes.
Remove from heat and let stand for several minutes.
Taste for seasonings and adjust accordingly.