In advance, peel and boil the potatoes and make mashed potatoes, or use leftover mashed potatoes or instant mashed potatoes (make according to package directions); set aside.
Cook vegetables according to package directions and set aside.
In a skillet, brown ground beef then sprinkle brown gravy mix over ground beef and no more than 1/2 cup of water just to thicken the gravy and make the ground beef a little moist - do not make ground beef soupy.
After ground beef has browned, evenly layer ground beef in a rectangular 9x13 casserole or baking dish; then evenly layer the cooked vegetables atop the ground beef; finally, spread the mashed potatoes carefully (I use a rubber spatula) over the vegetable layer until completely covered.
Sprinkle the French fried onions atop the mashed potatoes (optional), or arrange the pats of butter atop the potatoes if you do not use the onions.
Bake the Shepherds Pie for approximately 30 minutes, just to heat all the ingredients through.
Thermal Mechanics
Golden, crisp french fried onions contrast sharply against dense, steaming potatoes when baked at 350 degrees Fahrenheit for 30 minutes. Starch molecules in mashed potatoes lose structural integrity if overworked during mashing, releasing excess free liquid that ruins the underlying meat layer. Rubber spatulas distribute toppings evenly without dragging moisture up from the vegetable stratum.
Proportional Scaling
Halving this yield requires shifting from a 9x13 inch rectangular glass dish to an 8x8 inch square pan to maintain correct depth ratios. Meat thickness must remain under one inch so that thermal energy penetrates uniformly without drying out the protein. Excessive water added to gravy mixes breaks starch gelatinization, turning solid binders into a watery broth that compromises slice stability.
Structural Integrity
Moisture control prevents crusts from sliding off slick potato surfaces during baking. Butter pats melt downward, lubricating the starch matrix while creating localized browning spots across the potato top. Leftover starches require an extra five minutes under thermal exposure to compensate for initial refrigeration temperatures.