Preheat oven to 400°F. Coat a baking sheet or large cast-iron skillet with 1 tablespoon of grapeseed oil.
Place the onion and garlic into the bowl of a food processor fitted with an S-blade. Pulse a few times until coarsely chopped. Add the tempeh and pulse a few more times until finely chopped. Add the remaining 1 tablespoon of oil, panko, milk, soy sauce and pepper. Pulse until mixed, scraping down the sides of the bowl as needed.
Roll the mixture into 20 balls about 1 ½ inches in diameter. Place the meatballs on the baking sheet or skillet and bake, turning twice while they bake, until golden brown, about 20 minutes.
Make the sauce:
Place a medium saucepan over medium heat. Add the pineapple, broth, cider vinegar, brown sugar, tomato paste, soy sauce and Sriracha and stir to combine. Bring to a simmer, then lower heat to maintain the simmer and cook, stirring occasionally, for 10 minutes.
Combine the water and cornstarch to a small bowl and whisk until smooth. Add the cornstarch mixture to the sauce and stir just until combined. Add the meatballs to the sauce and cook, stirring frequently, until sauce becomes thick and coats the meatballs, 1-2 minutes. Transfer the meatballs to a serving platter and spear with decorative toothpicks for serving.
Tips:You can dice the pineapple as finely as you like. If you're serving these as an appetizer, you may want to cut the pineapple into about 20 chunks (or as many meatballs as you have) and skewer each chunk to the top of a meatball using a toothpick.If you begin your sauce about 5 minutes after placing the meatballs in the oven, you should be well timed for the meatballs to go into the sauce right after coming out of the oven.