Combine the panko, egg, ketchup, and Worcestershire sauce in a large bowl. Let it sit for 5 minutes so the panko fully absorbs the liquid.

Stir in the onion powder, garlic powder, salt, and pepper until evenly incorporated into the panko mixture.

Add the ground beef and mix gently just until combined. Scoop into 1 1/2-tablespoon portions and shape loosely into meatballs.

Heat the avocado oil in a large skillet over medium heat. Add the meatballs in a single layer and cook 7 to 9 minutes, turning every 1 to 2 minutes, until browned on several sides. Transfer to a plate and drain the excess grease, leaving the browned bits in the skillet.

Add the mushrooms to the hot skillet. Cook 5 to 6 minutes, stirring occasionally, until they release their water and the liquid fully evaporates.

Add the butter and diced onion. Cook 5 to 7 minutes, stirring occasionally, until the onion softens. Add the garlic and cook 30 seconds, stirring constantly.

Whisk the beef broth, brown gravy mix, Worcestershire sauce, and pepper together. Pour into the skillet, whisking while scraping up the browned bits from the bottom of the pan.

Bring the gravy to a gentle simmer. Cook 2 to 4 minutes, whisking frequently, until it begins to thicken.

Return the meatballs and any juices from the plate to the skillet, turning to coat in the gravy. Cover and simmer over low heat for 8 to 10 minutes, turning the meatballs halfway through, until their centers reach 155°F. Uncover and simmer 1 to 2 minutes more if needed, keeping the gravy slightly loose so it stays easy to portion.

If you're eating tonight, stop here. Serve the meatballs and gravy hot over your favorite base. Everything from this point on is for the freezer.

Let the meatballs and gravy cool for about 30 minutes. Divide into four portions using a 1-cup silicone mold cavity, filling each cavity about three-quarters full to leave room for expansion.

Let the filled portions cool until no longer steaming, then refrigerate about 30 minutes until fully cold. Cover once chilled and freeze on a level surface for at least 4 hours, or until frozen solid.

Pop the frozen portions out of the mold and transfer to a freezer bag or freezer-safe container. Press out as much air as possible before sealing. If vacuum sealing, freeze the portions solid first, then seal on a gentle or pulse setting to avoid crushing the meatballs. Label with the recipe name, freeze date, and portion size. Freeze for up to 3 months.

Place one frozen portion in a microwave-safe bowl, cover with a vented lid, and microwave at 50 percent power for 4 minutes. Separate the meatballs and stir the gravy, mushrooms, and onions. Microwave on full power 3 to 5 minutes more, stirring halfway through. Rest 1 to 2 minutes. Meatballs are ready when their centers reach 165°F.

Place one frozen portion in a small saucepan, cover, and heat over low heat for 5 minutes, or until the gravy begins to soften. Separate the meatballs and stir the gravy, then continue cooking over medium-low heat 6 to 8 minutes, stirring occasionally, until heated through.

Thaw one portion overnight in the refrigerator. Transfer to a microwave-safe bowl, cover with a vented lid, and microwave on full power 2 to 4 minutes, stirring halfway through. Rest 1 minute before serving.

Thaw one portion overnight in the refrigerator. Transfer to a small saucepan, cover, and heat over medium-low heat 6 to 8 minutes, stirring occasionally, until heated through.