Preheat the oven to 350°F.

Line a baking sheet with parchment paper. Set aside.

Mince the onion.

Set a small skillet over medium-high heat and pour in the olive oil. Once oil is shimmering, add the minced onion. Cook, stirring frequently, until onion is softened and beginning to brown, 5-6 minutes.

Transfer the cooked onion to a large mixing bowl. Add the ground beef, ground pork, eggs, panko, nutmeg, allspice, dry mustard, salt, and black pepper. Using clean hands, work the ingredients together until thoroughly combined.

Roll the meat mixture into 24 balls about 1 inch in diameter, or 1.5 oz. each, and place on the prepared baking sheet.

Bake the meatballs on middle rack of oven until cooked through, 16-17 minutes.

To make the sweet and sour sauce, place the pineapple juice, ketchup, soy sauce, brown sugar, and cayenne in a large pot and whisk until combined. Set the pot over medium-high heat and bring to a boil. Once boiling, reduce the heat to medium-low so that sauce is vigorously simmering. Simmer, stirring regularly with a rubber spatula, until sauce is thickened and reduced, for 10-15 minutes.

Check to see that meatballs are done. Remove from oven or add time as needed.

Transfer the meatballs to the pot of sauce. Simmer until heated through, 3-4 minutes.

Transfer meatballs and sauce to a large bowl and serve immediately.