Season chicken with salt, pepper, and paprika. Add olive oil to a deep skillet or Dutch oven over medium-high heat. Add chicken; cook and stir until no longer pink at the center, about 5 minutes. Remove chicken from the skillet to a plate; set aside.
Heat 1 ½ tablespoons butter over medium-high heat in the same skillet. Add mushrooms and onions; cook and stir until softened, about 5 minutes. Stir in garlic paste.
Add orzo and cook, stirring frequently, until orzo is lightly toasted. Pour in half the Marsala; cook and stir, until wine is nearly absorbed.
Add beef stock and bring to a boil. Reduce heat, then simmer until pasta is cooked and tender, about 10 minutes. Taste; season with salt and pepper as needed.
Add chicken, remaining Marsala, and remaining butter. Cook until chicken is heated through and liquid is mostly absorbed. Serve in wide bowls; garnish with parsley.
