Place six mini croquettes or ramekins on a baking tray and set aside.
Cut roast into 1 ½ inch squares and pat dry with a paper towel. Sprinkle liberally with kosher salt, onion powder, garlic powder, and pepper. Dust with flour and set aside.
In a large heavy-bottomed skillet with a lid (or a shallow Dutch oven), heat 2 tablespoon bacon fat over medium-high heat. Add mushrooms and stir to coat. Sprinkle with salt and cook until mushrooms release their juices and have browned. Remove from pan and add 2 tablespoon more bacon fat (or other cooking oil, if desired), working in batches, sear the roast on all sides before removing from pan.
Lower heat to medium and add onion and carrot, stirring to coat. Season with more kosher salt. Cook until carrots begin to soften and onions become translucent, about 5 minutes. Stir in garlic and cook until fragrant, about 1 minute more. Deglaze pan with brown ale, scraping all those flavorful bits up from the bottom. Stir in beef stock.
Return meat to pan and add a few sprigs of fresh thyme. Cover and cook over low heat for 1 ½ - 2 hours, or until liquid has reduced and meat is fork tender. Stir in frozen peas and cooked mushrooms. (At this point the filling can be cooled and refrigerated for a day, if desired.)
Preheat oven to 425 degrees. Divide filling among prepared ramekins. Cut the puff pastry into 6 equal squares (or using a large round cutter) to place over the top of the ramekins. Brush with eggwash and sprinkle with more fresh thyme.
Bake in preheated oven for 10 – 15 minutes, or until crust is golden brown. And serve hot!