While the noodles cook, heat 1/2 tablespoon butter in a large pan over medium heat. Add the sliced onion and cook, tossing, for about 5 minutes, until softened. Add the remaining garlic and cook for 30 seconds, until fragrant. Add stock concentrate and 1/2 cup water. Simmer until reduced by half. Stir in the sour cream and cook for 2 more minutes, until sauce has thickened slightly.