In a large frying pan over medium-high heat, add the ground beef and diced onion. Cook, stirring occasionally and breaking up the meat, until the beef is fully browned and the onion is softened. Drain any excess fat if needed. I like to season the beef lightly with salt and pepper at this stage for extra flavor.

Pour both jars of spaghetti sauce into the pan with the cooked beef and onion mixture. Stir well to combine and let it heat through for a few minutes on medium-low heat so the flavors meld together.

Spread 1 cup of the meat and sauce mixture evenly over the bottom of a 9x13 inch baking dish. This base layer prevents the noodles from sticking and keeps them moist during baking.

Place 4 oven-ready lasagna noodles over the sauce in the baking dish. Spread half of the remaining meat and sauce mixture evenly over the noodles, then sprinkle half of the shredded mozzarella cheese over the meat mixture.

Repeat the layering sequence: add 4 more oven-ready lasagna noodles, top with the rest of the meat and sauce mixture, and finish with the remaining shredded mozzarella cheese. Try to spread the cheese all the way to the edges so every slice is gooey and delicious.

Cover the lasagna tightly with foil and bake in a preheated 375°F oven for 30 minutes. After 30 minutes, remove the foil and continue baking for 15 minutes longer, or until the cheese is golden and bubbly on top. I like to let the lasagna rest for 10 minutes before cutting into it—this helps it hold together when served.