Wash and dry all produce. Preheat oven to 400 degrees. Halve, peel, and thinly slice onions. Core, seed, and dice tomatoes. Thinly slice baby gem lettuce. Drain and rinse beans. Mash with a fork or potato masher in a small bowl until mostly smooth but still a little chunky. Zest lime and cut into wedges.

Heat a large drizzle of oil in a large pan over high heat. Add beef, breaking it up into pieces with a spatula or wooden spoon. Cook until browned, 3-5 minutes. Season with salt and pepper. Remove from pan with slotted spoon and set aside. Carefully pour out and discard any excess grease from pan.

Add onions and a large drizzle of oil to same pan. Cook until browned, 6-7 minutes, tossing occasionally. Stir in Southwest spice blend. Season with salt and pepper. While onions cook, mix together sour cream, lime zest, and a squeeze of lime juice in a small bowl. Season with salt and pepper.

Stir crushed tomatoes and 1/4 cup water into pan. Season with salt and pepper, then bring to a simmer. Carefully ladle or spoon out about 1/4 cup of tomato mixture and set aside in a small bowl. Add beef and beans to pan and stir to combine. Let simmer until slightly thickened, 2-3 minutes.

Divide beef mixture evenly between tortillas to fill. Fold in the sides and roll up each. Place rolled tortillas in a row, seam-side down, in a large baking dish (you may need two). Pour reserved tomato mixture across the top. Sprinkle with cheddar. Bake in oven until cheese is melted, about 5 minutes.

While burritos bake, toss baby gem lettuce, tomatoes, a squeeze of lime juice, and a drizzle of oil in a large bowl. Season with salt and pepper. Divide burrito bake between plates, dollop with crema, and serve next to salad.