In a medium saucepan, combine the 2 cups of rice, 4 cups of water, and 1 tsp salt. Bring to a boil over high heat. Stir once, reduce heat to low, cover, and simmer for 18-20 minutes, or until water is absorbed and rice is tender. Remove from heat and let stand, covered, for 5 minutes before fluffing with a fork.
In a small bowl, combine the panko breadcrumbs and milk. Let sit for 5 minutes to allow the breadcrumbs to absorb the milk (this is called a panade and keeps the meatballs moist). In a large mixing bowl, combine the ground beef, ground pork, soaked breadcrumbs, beaten egg, minced onion, minced garlic, parsley, Worcestershire sauce, salt, and pepper. Use your hands to gently mix until just combined; avoid overmixing.
Preheat your oven to 400°F (200°C). Line a large, rimmed baking sheet with parchment paper or foil for easy cleanup. Using a tablespoon or a small cookie scoop, portion the meat mixture and roll into 1.5-inch meatballs (you should get about 24-28). Arrange them on the prepared baking sheet, spaced about 1 inch apart. Bake in the preheated oven for 20-25 minutes, or until browned and cooked through.
While the meatballs bake, make the sauce. In a medium saucepan, whisk together the pineapple juice, ketchup, rice vinegar, brown sugar, and soy sauce. Bring to a simmer over medium heat. In a small bowl, make a slurry by whisking the cornstarch with 2 tbsp cold water until smooth. Slowly whisk the slurry into the simmering sauce. Cook for 2-3 minutes, stirring constantly, until the sauce thickens and becomes glossy. Remove from heat and set aside.
In a large skillet or Dutch oven, combine the baked meatballs, thickened sweet and sour sauce, bell pepper pieces, and drained pineapple chunks. Gently stir to coat everything in the sauce. Place the skillet over medium-low heat and simmer for 8-10 minutes, just until the peppers are tender-crisp and the meatballs are heated through.
To serve, spoon a generous portion of the cooked white rice onto a white plate. Top with several sweet and sour meatballs, peppers, and pineapple, spooning extra sauce over everything. Garnish with sliced green onions and a sprinkle of sesame seeds, if desired.