Heat the oven. Preheat the oven to 400°F. Line a rimmed baking sheet with parchment paper and coat it lightly with olive oil spray. The parchment prevents sticking without adding fat, and it makes cleanup almost embarrassingly easy.

Soak the breadcrumbs. In a large mixing bowl, combine the panko and milk. Let it sit for 5 minutes until the panko absorbs the milk and turns soft. This mixture — called a panade — is the first moisture secret. It holds water inside the meatballs as they bake, which is exactly what lean turkey needs.

Mix the meatball mixture. To the bowl with the soaked panko, add the ground turkey, beaten egg, Parmesan, minced onion, garlic, tomato paste, parsley, Italian seasoning, salt, pepper, and onion powder. The tomato paste is the second moisture secret — it adds savory depth and extra juiciness. Mix with your hands just until everything is evenly combined. Stop as soon as it comes together; overworking the meat makes dense, tough meatballs. The mixture will feel soft and slightly wet — that is correct.

Shape the meatballs. With damp hands (wet hands prevent sticking), roll the mixture into balls about 1 1/2 inches across — roughly 2 tablespoons each. You should get about 20 meatballs. Arrange them on the prepared baking sheet with a little space between each one. Uniform size matters: it means they all finish cooking at the same time.

Bake. Bake for 18 to 20 minutes, until the meatballs are golden brown on the outside and reach 165°F on an instant-read thermometer. There is no need to flip them. If you want deeper browning on top, switch on the broiler for the last 1 to 2 minutes — just do not walk away.

Rest and serve. Let the meatballs rest on the pan for 5 minutes. This short rest lets the juices settle back into the meat instead of running out onto the cutting board — the third and final moisture secret. Serve warm over pasta with marinara, tucked into sub rolls with melted mozzarella, over a grain bowl, or as an appetizer with toothpicks and warm sauce for dipping.