Add 1 tsp canola oil to a large skillet over medium heat. Cook sausages until browned on all sides and internal temperature reads 160°F. Set aside on a covered plate while you make the onion gravy.***
Add another teaspoon of canola oil, sliced onions, salt, pepper, and minced garlic to the sausage skillet (no need to wash it first!) and cook until glossy and beginning to soften.
Add sage and beef broth to the skillet and whisk together to combine, integrating the bits of sausage and onions on the bottom of the skillet.
When your onions are nice and soft, after about 8 minutes, make a slurry to thicken the gravy to your liking. To make a slurry, spoon out a small amount of the hot beef and onion broth into a small bowl and whisk in some of the flour until a smooth, loose paste forms.
Pour the slurry back into the onion gravy and whisk to combine. Repeat the slurry step if you want your gravy thicker, but know that the heat from the pan will help it thicken up with a little patience. I would add 2 Tbsp of flour at a time, no need to exceed 1/8 cup.
You’ll know the gravy is the right consistency when it’s thick enough to coat the back of a spoon or takes a while to flow back to the center of the pan if you drag a spoon through it.
Scoop 1 cup of colcannon onto each plate and place 2 sausages on top with generous spoonfuls of gravy. Keep it classic and serve with cooked peas on the side!
*I got the large package of Kroger brand sausages on sale and froze the rest. I chose sausages with minimal added spices to let the traditional flavor of the onion gravy shine. However, you can choose whatever sausages you like best! I learned they need to be in the natural casing to be "bangers," though! During World War I and II in Britain, the sausages would often burst—or 'bang'— through the casing when cooked due to fillers like water added during meat shortages. The name stuck, and that's why they're referred to as bangers in this bangers and mash recipe!**Colcannon is a traditional Irish dish combining mashed potatoes, cabbage/kale, butter, and milk to create the creamiest, most flavorful mash you've ever tasted. You can also use regular mashed potatoes if preferred.***If you notice the sausage is browning too fast on the outside and not up to the correct temperature on the inside, add a tablespoon or two of water to the bottom of your pan and cover it. The steam will help the sausage cook through faster.