1. In a large frying pan, heat the oil over moderately low heat. Add the onion and sweet potato and toss to coat with the oil. Add the water, cover, and cook until the onions are soft, about 5 minutes. Stir in the sage, cayenne, salt, and tomatoes. Reduce the heat and simmer, covered, until the sweet potato is tender and the sauce thickened, about 15 minutes. Add the Canadian bacon and remove from the heat.
2. In a large pot of boiling, salted water, cook the bow ties until just done, about 15 minutes. Drain and toss the pasta with the sauce.
Variations: Bow Ties with Sweet Potatoes and Ham: Dice 1/2 pound of ham and use it in place of the Canadian bacon.
Bow Ties with Canadian Bacon and Squash: Peel and dice 1/2 pound of winter squash, such as butternut or Hubbard, and use it in place of the sweet potato. Depending on the type of squash, you may need to adjust the simmering time by a few minutes.
Wine Recommendation: To match the sweetness of both the potato and the Canadian bacon, try a kabinett riesling from Germany's Mosel-Saar-Ruwer region.