Nuke ’em if you don’t have time to roast. I like to dry-roast butternut squash before mashing to intensify the flavor, but yams are so intense as it is that microwaving works fine.
Start with 5 minutes on high for one small yam. Add 2-3 minutes for each additional yam. I scrub them and toss them in on a plate.
Keep cooking until really mushy. Then break in half and let cool for a bit, peel the skin off by hand, scraping off any extra that sticks to the skin, add some olive oil, salt, pepper, a bit of maple syrup, and mash. Then, gradually add bits of water at a time while mashing until it is the consistency that you like. Keeps warm well in a double boiler if you want to make it ahead of time. An easy and healthy veggie that goes with all sorts of meals.