Prep the Potatoes and Carrots
Peel and cut potatoes into chunks. Place in a large pot of salted water and bring to a boil. Cook until tender, about 15-20 minutes. While potatoes are cooking, peel and chop carrots small, then cook in a separate pot for 15 minutes. Drain carrots but save some of the cooking water.
Cook the Mince
In a large frying pan, fry chopped onion for a couple of minutes until softened. Add mince beef and turn up the heat to cook well, about 20 minutes. While the mince is cooking, add the oxo cubes, black pepper, dried mixed herbs, and all-purpose seasoning if using. Keep stirring to make sure it doesn't burn—turn down heat if needed.
Combine the Filling
Microwave frozen peas according to package directions and drain. When mince is nearly cooked, add the cooked carrots and peas. Add a little of the reserved carrot water (about 1/4 cup) and stir in the Bisto gravy granules to thicken. Continue cooking on low for a few more minutes with lid on if desired.
Make the Mash
Drain the cooked potatoes and return to pot. Add butter and milk, then mash until smooth and creamy. Season with salt and pepper to taste.
Assemble and Bake
Preheat oven to 400°F (Gas Mark 6). Transfer the mince mixture to a baking dish. Spread the mashed potatoes evenly over the top. Sprinkle generously with grated Red Leicester cheese.
Bake Until Golden
Bake for about 40 minutes until the top is golden and the filling is bubbling around the edges. Let rest for 5 minutes before serving. Enjoy with crusty bread or on its own!