This is a perfect candidate for meal prep. You can prepare the Meat Base up to 2 days in advance and keep it refrigerated. When you’re ready for dinner, simply boil the potatoes, top the cold meat, and add 5–10 minutes to the bake time. It actually tastes better on day two as the umami flavors in the bouillon and soy sauce continue to marry.
Don't fear the fat. Using 80% lean ground beef provides the moisture and "mouthfeel" that leaner blends lack. We use 3 tablespoons of tomato paste specifically to emulsify those beef fats into a silky gravy, ensuring the dish is rich but never greasy.
To ensure a structured topping that doesn't "sink" into the meat, add your milk slowly. Depending on the starch content of your Russets, you may not need the full 1/4 cup. You want a mash that is creamy enough to spread but firm enough to hold its shape under the heat of the oven.