Preheat the oven to broil at 400° F.
Peel and cube potatoes (if you like the skins on your potatoes, do not peel). Boil until fork tender — do not let them get mushy. Drain water off and mash with potato masher or a stand mixer. Stir in butter while the potatoes are still hot. Next add pepper (begin with ¼ teaspoon), salt (begin with 1/2 teaspoon) and garlic salt (begin with 1/4 teaspoon) – taste before adding too much–you can always add more, but you can't take it away!
Mix eggs, if using, in a separate bowl, then stir into potatoes, mixing well. Begin adding milk until the mash reaches your preferred consistency (add slightly more, as the potatoes will dry out a bit in the oven). Try not to over mix the potatoes, as they can tend to get very thick and starchy. Once prepared, cover with foil and set aside.
Prepare the lentils according to package directions. Once cooked, and all the water is absorbed, stir in one cup of soup. Set aside.
Prepare vegetables and toss lightly with olive oil. Distribute onto two large jelly roll or rimmed baking sheets lined with foil. Roast in the preheated oven for 15 minutes, toss, and continue roasting until all vegetables are fork tender, approximately 25 minutes in total.
Remove from the oven and reduce the oven temperature to bake at 350° F.
Stir in fresh herbs, salt and pepper with the roasted vegetables.
Prepare a 3 quart baking dish or 6-8 ramekins by oiling well. Line dish(es) with an even layer of cooked lentils, then layer with vegetables and top with mashed potatoes.
Bake until the mashed potatoes are lightly browned and the filling is bubbly, approximately 30 minutes.
Serves 8.