Preheat oven to 400 degrees F, and line a baking sheet with foil. Wash and dry the sweet potatoes and prick each one a few times with a fork.
Place sweet potatoes on baking sheet and bake for 1 hour, flipping them at the halfway point, or until a knife slides in easily. Let cool for about 20 minutes or until cool enough to handle. Alternatively, sweet potatoes can be baked 1 to 2 days in advance and refrigerated until ready to peel and proceed with the recipe.
Toast the pecans and saute the veggies:
Preheat a saute pan over medium-low heat. Toast pecans in the pan, stirring occasionally, for about 5 minutes or until fragrant. Transfer pecans to a plate to cool, then chop and set aside.
Return saute pan to the burner and increase heat to medium. Cook the onion for about 5 minutes or until translucent, stirring occasionally. Add broth and garlic to the pan, and cook 3 to 4 minutes. Add rosemary, sage, and pepper, and cook until most of the broth has evaporated and onions are softened. Remove from heat.
Assemble the dish:
Peel the sweet potatoes, discarding the skins, and place in a large bowl. (If potatoes were made in advance and are still cool, warm them in the microwave or on the stovetop.) Add about ½ cup yogurt to the bowl, and use a potato masher or fork to mash the sweet potatoes. Add onion mixture and salt, and stir to combine. Taste and add more yogurt, salt, and pepper, as desired.
Transfer mashed sweet potatoes to a serving dish. Garnish with a swirl of yogurt and chopped pecans.
I don't usually specify whether to use organic ingredients or not, but I find the flavor difference between organic and conventional sweet potatoes to be significant. So I thought it's worth mentioning here that organic sweet potatoes will create the best tasting dish.
I used homemade soy yogurt in the recipe during testing, but other plain vegan yogurt is fine, too, such as cashew or almond. I prefer the flavor and texture of homemade yogurt (check out this post to learn how to make yogurt at home), but store-bought will also work. Just make sure it's 100% unsweetened and unflavored.
Make this recipe nut-free by using soy yogurt and substituting toasted pumpkin seeds for the pecans.
Store leftover sweet potato mash in a covered container in the refrigerator for up to 4 days. To freeze, let cool to room temperature then transfer to a freezer-safe, airtight container.