Set out a standard 9" x 5" loaf pan. (Line with a strip of parchment paper for easy removal if desired.)
Add eggs, ketchup, Italian seasoning, kosher salt and pepper to a large bowl and whisk until well combined. Stir in onions, mushrooms and oats. With your hands mix in ground beef, just until combined. (Don't over-mix or your meatloaf will be tough.) Transfer mixture to loaf pan and gently press to fill; smooth the top with a spatula or the back of a spoon.
Bake until the internal temperature reaches 160ºF, 1 to 1 ¼ hours. Allow to rest for 10 minutes before serving. Slice and serve.
Slicing tip: After your meatloaf has cooled a bit, use two metal spatulas, one on each end of the loaf pan to carefully lift the meatloaf out and transfer to a cutting board for slicing. Alternatively, you can line your loaf pan with a strip of parchment paper before pressing in the meatloaf mixture for easy removal.