Preheat the oven to 425 degrees F. Scrub the sweet potatoes and prick them with a fork about 9 times all over the surface.
Bake the potatoes for 50 minutes to 1 hour, or until a fork easily pierces through. Remove them from the oven to cool for about 15 minutes, or until they can be easily handled.
Scoop out the flesh into a bowl and puree with an immersion blender, or mash very well with a meat masher (or a fork) until as smooth as possible. Set aside.
Warm the stock in a separate saucepan over medium-low heat and maintain the heat. (You want your stock very warm, but no need to boil.)
In a 5-quart Dutch oven or large saucepan over medium heat, heat the oil and butter until the butter has melted and is hot. Do not allow the butter to brown.
Add the onions and a pinch of salt and pepper, to taste. Keep in mind the saltiness of the stock and the Gorgonzola that will be added at the end.
Stir the onions often, sautéing just until softening and turning opaque. Turn the heat down if one or two look like they are browning.
Add the rice and stir often to “toast” the rice grains. Once most of the risotto rice is turning opaque and smelling quite fragrant, add the garlic and stir for about 20 seconds.
Begin adding the stock, 1 cup/ladleful at a time, stirring until the rice has absorbed the liquid. Continue adding additional stock, 1 cup at a time, until the risotto is creamy and the grains are soft with a bit of “bite” still in the center, but not “crunchy.”
The cooking process up till now could take about 35 to 45 minutes. Your patience will be rewarded.
Once the risotto is cooked perfectly, add about ½ cup more of the warm stock. Stir and heat through.
Crumble the Gorgonzola over the risotto and stir until melted.
Add the pureed sweet potato and combine well. Add more stock as needed if the mixture is too thick for you. It should be creamy.
Taste and adjust any salt and pepper as desired.