Heat olive oil in a skillet over medium-high heat; add chicken and season with salt and pepper. Cook and stir until chicken is no longer pink in the center, 5 to 10 minutes. Transfer chicken to a plate.
Melt butter in the same skillet; cook and stir mushrooms and garlic until mushrooms are lightly browned, 5 to 7 minutes. Mix tomato and chicken into mushroom mixture; season with salt. Cook and stir, 2 to 3 minutes. Add spinach and gently stir until wilted, 2 to 3 minutes.