In a dutch oven, or similar, brown ground beef and break into small pieces with a wooden spoon or spatula.
1 pound 85/15 ground beef
Add the onion, garlic, bell pepper, and seasonings. Stir and cook until softened, about 5-7 minutes.
1/2 large onion, 2 cloves garlic, 1 medium red bell pepper, 2 teaspoons chili powder, 1/2 teaspoon ground cumin, 1/2 teaspoon smoked paprika
Add tomato sauce, beef stock, and pasta shells. Stir to incorporate then bring to a low simmer. Cover, stirring occasionally, cook until pasta is done.
1 1/4 cup tomato sauce, 1 1/4 cup beef broth, 2 cups dried medium shells pasta
Remove from heat and add Velveeta and stir to melt and incorporate. Season with salt, to taste.
4 ounces Velveeta, Kosher salt, Ground black pepper
Notes
You may need more or less liquid depending on the size of your pasta shells or pasta shape used. I used "medium" pasta shells but "medium" is a relative term and totally dependent on what the grocery store/brand considers "medium". You're looking for a thick sauce covering the pasta, not a loose liquidy sauce.