In a small bowl, whisk together the mayonnaise, dill, paprika, onion powder, garlic powder, and red pepper flakes. Set aside.
Slice the sourdough into thick slices, about ½ to 1 inch wide.
Preheat oven to 400°F (or use a toaster oven).
Season both sides of the bison medallions with salt and pepper. In a skillet over medium-high heat, melt the butter until frothy. Sear the medallions 2–3 minutes per side for medium-rare (or longer for desired doneness). Remove from heat and let rest for 10 minutes.
Spread the mayo mixture onto one side of each bread slice. Place the slices mayo-side up on a baking sheet and toast in the oven for 5–8 minutes, until golden and crisp.
Thinly slice the rested bison medallions against the grain.
Assemble the sandwiches with the toasted bread, sliced tomato, avocado, lettuce, and bison. Serve immediately. Enjoy!
This recipe was created in collaboration with The Starving Chef. You can like her on Facebook and subscribe on YouTube for weekly recipes. You can find this recipe, and many others, with step-by-step pictures on The Starving Chef Blog.