In a sauté pan over medium-high heat melt first amount of butter ( 3 Tbsp ). Add sliced onions and cook, stirring often, until well browned, about 10-12 minutes. Reduce heat to medium if needed for even browning of onions
Add second amount of butter ( 1 Tbsp ) to pan along with the cremini mushrooms and cook until softened. Add garlic powder and thyme and cook until fragrant.
Add pasta, beef stock, cream, and salt. Bring to a boil then reduce heat to medium-low and simmer. Cook until pasta is cooked al dente, about 15 minutes.
Stir in half of grated cheese to cooked pasta. Portion pasta into 8 oz ramekins. Hold warm for service.
For each serving, sprinkle 1/4 cup cheese over one ramekin. Place under salamander and cook until cheese begins to brown. Sprinkle with 1/8 teaspoon chives. Serve hot.