Put the sliced onions, olive oil, butter, and a pinch of salt into the pot. Turn the heat to medium and cook the onion, stirring occasionally, until it's very soft. The onions should be completely wilted and a tawny gold color.
When the onions are just about done, warm the broth in a sauce pan or in the microwave. Keep it at a simmer or reheat in the microwave as necessary while cooking the risotto. Temperature is important for cooking properly.
Increase the heat on the onions to medium-high and stir in the rice. Turn it thoroughly, making sure every grain comes in contact with the onions. You want the rice to take on that color and shine.
Add 1 cup of the warm broth and stir well, scraping down the sides and bottom of the pot as you go. You don't want any grains to stick. When the liquid is gone, add more, about ½ cup at a time. Stir and repeat until the liquid is used up and the rice is tender but still ad dente. The process takes about 25 minutes.
Remove the pot from the heat. Stir in the butter, Parmigiano-Reggiano, and sage strips. Add a bit of salt and pepper to taste. Serve immediately.
Slow Cooker Instructions
In a 3-quart slow cooker, combine the onion, olive oil, butter, salt, stock, and arborio rice in the slow cooker. Cook on High for 2 to 2.5 hours. Stir in the Mantecare and cook uncovered for another 15 minutes.