Start a pot of water for the pasta. Put the cubed sweet potato in a lidded, microwave-safe container. Add 2 teaspoons of water and microwave for 2 minutes with the lid vented. Test sweet potato with the tip of a sharp knife; the knife should insert easily. You want the potato just tender, but not mushy. If not, microwave it for an additional minute and test again.
Heat the oil in a large skillet over medium flame. Drain sweet potato and sauté for 2 to 3 minutes, stirring and turning occasionally. Add onion and toss to coat. If using dried sage, add to pan now. Cook for a minute or two, stirring occasionally.
Add chicken to pan. Season generously with salt and pepper and cook until chicken is just cooked through, stirring occasionally, 5 to 6 minutes. Add garlic to pan and cook until just fragrant, about 45 seconds. Remove from heat.
Meanwhile, cook pasta according to package directions. Drain, reserving ¼ cup cooking water. Add to skillet with sweet potato mixture and toss to combine. If the dish seems dry, add a little reserved pasta water, a tablespoon at a time.
If you’re using fresh sage, sprinkle with ⅔ of the sage and toss to combine. Divide among shallow pasta bowls and top with remaining sage. Serve immediately.