Preheat oven to 400 degrees Fahrenheit.

Cook spaghetti until al dente, about eight minutes. Drain and rinse under cold water.

In a large bowl, toss spaghetti with eggs, salt, and garlic and mix to combine.

Spray a muffin tin with cooking spray and nest spaghetti inside. Using the bottom of a shot glass, press down firmly to create a well.

Bake until set, about 15 minutes. Let cool. (If nests puff up while baking, press down again with shot glass.)

Combine tomato sauce, brown sugar, vinegar, ketchup, soy sauce, and four cups pineapple juice in a large saucepan over medium-high heat. Bring to a boil.

Mix cornstarch and remaining quarter cup pineapple juice in a small bowl until fully dissolved. Slowly stir it into the sauce until incorporated. Bring sauce to a boil, then reduce heat and let simmer.

Mix bread and grated onion and let sit until bread disintegrates, about five minutes.

Add ground beef, egg, garlic, salt, and pepper. Use your hands to mix well.

Measure a heaping tablespoon of meat mixture and roll lightly between the palms of your hands to form a ball, then drop it into the pot of simmering sauce. Repeat with the remaining mixture.

Cover and simmer for 30 minutes.

Serve meatballs in spaghetti nests.