Bring a large saucepan of water to a boil. Add pasta and cook according to package directions; drain.
Sprinkle chicken with garlic powder and 1/4 tsp. each salt and pepper. Place flour in a shallow bowl and coat both sides of the chicken, shaking off any excess. Reserve 2 tsp. flour; discard the rest.
Heat 2 Tbsp. oil in a large skillet over medium-high heat. Cook the chicken, turning once, until golden brown and cooked through, 5 minutes per side. Transfer to a clean plate.
Reduce heat to medium and add the remaining 1 Tbsp. oil to the pan. Add shallot and cook, stirring, until beginning to brown, about 1 minute. Add the wine and cook, stirring occasionally, for 1 minute. Combine water with the reserved 2 tsp. flour. Add to the pan and cook, stirring, until thickened, about 1 minute. Remove from the heat; stir in sour cream, mustard, 2 Tbsp. sage and the remaining 1/4 tsp. each salt and pepper. Return the chicken to the pan and turn to coat with the sauce.
Top the pasta with half the sauce, the chicken, and then the remaining sauce. Garnish with some more sage, if desired.