Preheat the oven to 375 degrees F. Lightly coat a 1 ½ quart baking dish with cooking spray.
Place the sweet potatoes in a large saucepan and cover with cold water. Bring to a boil and cook until the potatoes are tender when pierced with a fork, about 10 minutes.
Drain and return the potatoes to the saucepan.
Mash the potatoes with the butter, milk, salt and pepper until smooth.
Heat the olive oil in a large nonstick saucepan set over medium-high heat.
Add the onions and cook until they are starting to brown, about 4 minutes.
Add the ground bison. Cook, crumbling with a fork, until the meat is cooked through.
Add the tomato paste, garlic, thyme, rosemary, salt and pepper, and cook for 1 minute.
Stir in the broth, reduce heat slightly and simmer until most of the liquid is absorbed, about 12 minutes. Stir in the peas.
Transfer the filling to the prepared baking dish.
Dollop the sweet potatoes on top of the filling and, using a rubber spatula, spread the potatoes evenly on top, all the way to the sides. Be sure that the potatoes are sealed at the sides of the baking dish so that the filling doesn’t bubble through while cooking. Using a fork, make hatch marks on the potatoes, if desired.
Place the baking dish on top of a baking sheet.
Bake until the potatoes are just starting to brown, about 30 minutes.