Heat a medium frying pan or skillet over medium-high heat. Add the onions and cook for 2-3 minutes to soften them. Stir frequently. If the onions start to stick, add a tablespoon of water.
Add the garlic and mushrooms, stirring frequently, and cook for 3-4 minutes until the mushrooms soften and release their moisture, allowing any excess water to dissipate. Remove from the heat.
In a food processor with a metal blade, grind 1 cup of oats until they break down. They don't need to be as fine as flour.
Add the onions and mushrooms to the oats, then add the chickpeas, ¼ cup sour cream, ¾ teaspoon allspice, ½ teaspoon nutmeg, 1 teaspoon salt, ½ teaspoon pepper, and 2 tablespoons parsley. Pulse to combine the ingredients, leaving a little texture. After the first few turns, use a spatula to scrape the sides and bottom.
To test the meatball mixture, form a small ball with your hand. If it crumbles, continue blending a few more turns. Add another teaspoon of sour cream if needed. If the mixture is too moist and melts, into your hand, add a tablespoon or more of ground oats, or add chickpea flour or another flour.
Preheat the oven to 400 F (200 C). Line a baking tray with parchment paper.
In a small bowl, whisk the cornstarch with ½ a cup of water until it is smooth. Slowly add half of the mixture to the gravy and stir until it thickens. Add more until the sauce has thickened to the desired consistency.
Make 16-18 small balls and place them on the baking tray, separated and in a single layer. I used about 1 ½ tablespoons of meatball mixture per ball.
Place the meatballs in the center rack of the oven. Bake for 20-25 minutes until the meatballs are firm and golden brown.
To make the gravy, combine 1 cup of vegetable broth with 2 tablespoons of cornstarch, forming a smooth slurry.
In a medium saucepan, add 3 cups of vegetable broth, ½ cup vegan sour cream, 1 tablespoon mustard, ⅛ teaspoon allspice, ⅛ teaspoon nutmeg, ½ teaspoon salt, and ½ teaspoon pepper. Set on medium heat and whisk the ingredients until they start to bubble.
Add the cornstarch slurry and continue whisking and simmering on low until the gravy thickens. Stir in the parsley. Taste and adjust the spices as needed.
Serve Swedish meatballs and gravy over pasta or mashed potatoes.