In a dutch oven, or similar, brown ground beef and break into small pieces with a wooden spoon or spatula.
Add the onion, garlic, bell pepper, and seasonings. Stir and cook until softened, about 5-7 minutes.
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.
Remove from heat and add Velveeta and stir to melt and incorporate. Season with salt, to taste.
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.