Heat a large pot** with oil over medium-high heat. Add diced onion and garlic, sauté together until slightly tender 3 to 4 minutes.
Add in black beans, rotel (include liquid), and taco seasoning. Mix everything to combine. Add in dry spagetti (break in half for ease) and 3 ½ cups water. Cover and cook 10 minutes.
Remove cover and stir well ensuring nothing is sticking to the bottom. Continue to cook uncovered 5 to 8 minutes stirring occasionally until spaghetti is cooked to your liking and most (but not all) of the liquid is cooked down.
Turn off heat. Stir in the cheddar cheese and sour cream. Transfer to serving dishes, garnish with green onions, and serve immediately.
