Preheat the oven to 325°F (160°C). Line a baking sheet with parchment paper and set aside.
Bring a large pot of salted water to a boil and cook the broken spaghetti according to package directions until al dente; drain and set aside.
In a 5–6 quart saucepan over medium-high heat, add the ground beef and cook uncovered, stirring often, until no longer pink, about 5–7 minutes; drain excess grease.
Sprinkle the mild taco seasoning over the cooked beef, pour in 1/2 cup water, stir to combine, lower heat to medium, and cook 3–4 minutes, stirring occasionally.
Reduce heat to low and stir in the 2 cups of spaghetti sauce until heated through; then remove the pan from the heat.
Add the cooked spaghetti to the meat and sauce mixture and use tongs to toss until the noodles are fully coated.
Place the stuffed hard taco shells evenly on the prepared baking sheet and bake 5–6 minutes to warm and crisp them slightly; remove from oven.
Fill each taco shell about two-thirds to three-quarters full (roughly 1/3 to 1/2 cup) with the spaghetti mixture, then top with grated Parmesan and diced Roma tomato if using; serve immediately.