In a large skillet or pot over medium heat, add the ground beef and Italian sausage. Break the meat apart as it cooks and continue cooking until it is no longer pink.
Season with salt and pepper. Add the diced onion directly to the pan and cook for about five minutes, or until the onion has softened and become slightly translucent.
Sprinkle in garlic powder and stir in the minced garlic. Cook for about one minute, just until fragrant. Be careful not to let the garlic burn.
Add the tomato paste directly to the meat mixture. Stir frequently and allow it to cook for four to five minutes. You will notice it darken in color and thicken slightly. This caramelizing step is the key to building rich, concentrated tomato flavor in a short amount of time.
Pour in the jar of marinara sauce and stir well to combine.
Add one quarter cup of water to the empty marinara jar, secure the lid, and shake to loosen any remaining sauce. Pour this into the pot and stir again.
Reduce the heat to low and let the sauce gently simmer while you cook the pasta. Even ten to fifteen minutes of simmering will allow the flavors to meld beautifully.
In a separate pot, bring salted water to a boil and cook the spaghetti according to package directions until al dente.
Drain the pasta and add it directly into the sauce. Toss thoroughly so every strand is coated.
Serve warm.