Heat the oil in a large saute pan over medium heat until shimmering. Add the onions and red and green bell peppers, season with salt, and cook, stirring occasionally, until softened, for about 6-8 minutes.
Add the garlic, chili powder, cumin and red chili pepper flakes, stir to coat the vegetables, and cook until fragrant, for about 1 minute. Add the ground beef, being careful not to break it up too much and cook until the beef is no longer pink, for about 7 minutes.
Carefully transfer the mixture to your slow cooker, add the diced tomatoes and their juices, the tomato sauce, pour in the beer and beans, and stir to combine. Cover and cook until the chili thickens and the flavors meld, for about 8 hours on low or 6 hours on high.
Taste and season with salt as needed, and serve with sour cream, loads of cheddar cheese and garnish with fresh chives. Enjoy!