Heat oven to 325°F. Season lamb with 1/2 teaspoon salt and pepper. Heat oil in a covered oven-safe pot and brown lamb in two batches for 3 minutes per side, then lift out.
Soften leeks and carrots in the pot for 6 minutes. Stir in flour for 1 minute, gradually add broth while scraping the bottom and return lamb with thyme.
Bring to a simmer, cover and braise in the oven for 2 to 2 1/2 hours until the lamb pulls apart easily. Meanwhile, prick sweet potatoes, place on a sheet pan and bake alongside the covered pot for 75 to 100 minutes until very soft.
Lift lamb from the gravy and shred into short pieces, discarding any hard fat. Simmer the gravy uncovered on the stove for 5 to 10 minutes until thick, then fold in lamb and beans.
Scoop sweet potato flesh from its skins and mash with butter, buttermilk and remaining salt. Raise oven to 400°F; divide filling among six 2-cup oven-safe dishes and spread equal amounts of mash over each.
Bake for 20 minutes until the filling bubbles and reaches 165°F. Cool promptly in shallow dishes and refrigerate within 2 hours; once chilled, cover tightly and freeze in freezer-rated containers for up to 3 months.
To reheat one frozen pie, use a microwave-safe container rather than frozen oven glass. Add its measured water down one side, cover loosely and microwave at 50 percent power for 5 minutes, then at full power in 1-minute intervals, rotating and checking several central spots, until all reach 165°F, usually another 3 to 5 minutes.
Rest for 2 minutes and check the center once more before eating. Keep the topping over the filling during heating to hold in moisture.