Place bison and angus beef into a mixing bowl.
With a large cooking fork, mix the two portions of meat together.
Divide mixed meat into four portions, and then mold each portion into burger patties. Do not overwork the meat, as too much handling can cause toughness once cooked.
Remember the rule of making sure the center of each burger patty is thinner in the center than the outer edges. Once you cook each one you will notice that the center expands.
Season each patty with salt and pepper.
Heat skillet and when hot add 1 1/2 tbsp of butter.
Cook patties on a high to medium heat until crust develops, add the remaining 1/2 tbsp butter then flip over to cook other side.
Allow a crust to develop on the second side, and you should have a nicely cooked medium burger. If you prefer your burger cooked to a warmer temperature I would suggest you finish off in the oven, at 375.
Remove patties from skillet and place on a board.
Top each patty with Stilton and tent with aluminum foil and allow to rest.
Place burger buns cut side down into skillet on a high heat to heat through and soak up some of the juices from the burgers.
Place spinach and onion in a bowl and toss to mix and then drizzle with balsamic vinegar.
Assemble burgers by first adding a layer of spinach on top of bottom half of bun. Add burger with cheese on top then cover with top half of bun.
Serve immediately.