Season the pork cubes with salt and pepper. Sear them over medium-high heat, then set aside, leaving oil in the pan.
Turn the heat down to medium and add the onion. Cook and stir until slightly browned, about 5 minutes. Then stir in the garlic and cook until fragrant, about 1 minute.
Add the chicken broth and sour cream. Whisk until smooth. Scrape up any bits at the bottom of the pan. Bring the mixture to a simmer.
Return the pork to the pan, along with any juices the pork gave up while sitting. Add the sage as well.
Turn heat down to low and simmer covered until the meat is almost tender, about 1 hour.
Then raise the heat to medium and simmer uncovered until the pan sauce thickens and the meat is very tender, about another 20 minutes.
This is amazing over polenta–the flavor matches polenta much better than other options like potatoes or rice. So I always make it with polenta!