Wash and dry all produce. Preheat oven to 400 degrees. Bring 1 cup water, stock concentrate, and a pinch of salt to a boil in a small pot. Halve, peel, and dice onion. Trim, then thinly slice scallions. Cut lime into wedges.

Once water is boiling, add quinoa to pot. Cover, lower heat, and reduce to a simmer. Cook until tender, 15-20 minutes.

Heat a drizzle of olive oil in a large pan over medium-high heat. Add onion and scallions and cook, tossing, until softened, 4-5 minutes. Season with salt and pepper. Add beef, breaking up meat into pieces. Cook until browned, 3-4 minutes. Stir in Southwest spice and cook until fragrant, about 30 seconds. Season with salt and pepper.

While onion and beef cook, halve bell peppers lengthwise, then remove white ribs and seeds. Rub all over with a drizzle of olive oil and season with salt and pepper. Place on a baking sheet. Roast in oven until softened, 5-7 minutes. Once cool enough to handle, arrange bell peppers cut-side up on baking sheet.

Stir tomatoes into pan with beef and bring to a simmer. Let simmer until flavors have come together and tomatoes have reduced slightly, about 5 minutes. Once quinoa has finished cooking, add it to pan and stir to combine. Season with salt and pepper.

Stuff bell peppers with as much filling as will fit. Sprinkle Monterey Jack cheese over top of each. Return to oven and continue roasting until very soft, about 10 minutes. Divide remaining filling between plates, then place stuffed peppers on top. Serve with lime wedges on the side for squeezing over.