Brown the beef in olive oil in a large pan over medium-high heat with salt and pepper. Break up and cook until fully browned.
Add vegetables: Add onion, carrot, celery, zucchini, and garlic. Sauté for 2–3 minutes until softened.
Add tomato paste, Worcestershire sauce, bone broth paste, paprika, thyme, sweetener, and flour. Stir well and sauté for 2–3 minutes.
Add stock, bring to a boil, then reduce heat and simmer for 30 minutes until thickened, adding frozen peas halfway through.
Divide filling evenly between 4 oven-safe bowls or ramekins.
Boil potatoes until tender, drain well, and mash with bone broth paste or butter, milk, salt and pepper until smooth and creamy.
Spoon or pipe mash on top of each bowl. Top with mozzarella. Bake at 190°C for 20–25 minutes until golden and bubbling.
Serve immediately or cool completely before covering and refrigerating or freezing.