Preheat the oven to 325°F and line a baking sheet with parchment paper.
Cook the spaghetti according to package directions; drain and set aside.
In a 5–6 quart saucepan over medium-high heat, add the ground beef and cook, stirring often, until no longer pink, about 5–7 minutes; drain any excess grease.
Sprinkle the taco seasoning over the cooked beef, pour in ½ cup water, stir to combine, then reduce heat to medium and cook 3–4 minutes, stirring often.
Reduce heat to low and stir in 2 cups spaghetti sauce until heated through; remove from heat.
Arrange the stuffed hard taco shells evenly on the prepared baking sheet and bake 5–6 minutes to warm; remove and set aside.
Add the cooked spaghetti noodles to the meat sauce and use tongs to toss until the noodles are fully coated.
Spoon about ⅔ to ¾ full (roughly ⅓ to ½ cup) of the spaghetti mixture into each taco shell, then sprinkle with grated Parmesan and diced Roma tomato if using; serve immediately.
