Add the sun-dried tomatoes and walnuts to a food processor and pulse into a paste-like texture. Add the remaining ingredients and process until a crumbly dough forms, stopping to scrape down the sides, as needed.
Once your dough is formed, pinch off walnut-sized pieces and roll them into a ball. Place the meatballs in the steamer basket as you finish them. You should end up with about 20 meatballs.
Next, add a few inches of water to the steamer pot (which could be a bamboo steamer or a large pot with a steamer basket insert) and bring to a boil. Once the water begins to boil, add the steamer basket with meatballs, cover and steam for 30-45 minutes, or until firm (see Notes). To ensure even cooking, turn the meatballs halfway through cooking. The meatballs will expand a bit when they steam.
Once your meatballs are done steaming, you can use them right away, store them in the refrigerator for up to five days, or freeze them for later use.
When you're ready to use them, add a tablespoon of olive oil to a skillet over medium-high heat. Add the meatballs and fry a couple of minutes per side, turning as needed until they're heated through and have a nice crust on the outside.