How to Make It
Preheat oven to 450°F. Heat a large (12-inch) cast-iron skillet over medium-high. Add lean ground beef sirloin. Cook, breaking up and stirring with a wooden spoon, until well browned, 6 to 8 minutes. Add pearl onions, fresh thyme leaves, minced garlic, black pepper, and 3⁄4 tsp. kosher salt, and stir until combined. Cook until fragrant, 2 to 3 minutes. Stir in tomato paste, and sprinkle with all-purpose flour. Cook, stirring often, until well incorporated, about 2 minutes. Add peas and carrots; stir to combine. Stir in beef stock; bring to boil. Reduce heat to medium-low; simmer until thickened, 15 to 20 minutes.
Meanwhile, whisk together self-rising flour and 1 tsp. kosher salt in a large bowl. Using your hands, work unsalted butter into mixture until it clumps together and becomes pea-size pieces. Stir in 1 cup shredded mild Cheddar cheese. Stir in whole buttermilk just until incorporated.
Using a 1⁄4-cup ice-cream scoop, drop biscuit dough directly on top of stew, leaving about 1⁄2 inch of space between biscuits. Sprinkle top with 1⁄2 cup shredded mild Cheddar cheese.
Place skillet on a rimmed baking sheet, and bake in preheated oven until biscuits are browned and flaky, 20 to 22 minutes. Let stand 5 minutes, and serve.