Mix beef, grated onion, egg, breadcrumbs, non-dairy milk, salt, and pepper and spices. Form small balls (~1 inch diameter). Chill them in the fridge for about 1–2 hours to help them hold their shape.
Preheat the oven to 180°C.
Brown the meatballs in batches in a frying pan with a little oil until all sides are golden. Transfer to an oven and bake at 180°C for about 15–20 minutes to finish cooking evenly.
In the same pan, melt the butter and whisk in the flour to make a roux. Gradually add the stock while whisking until smooth. Stir in the soy sauce and the mustard. Add the cream and heat until starting to thicken.
Pour the sauce over the meatballs or return meatballs to the sauce and simmer briefly. Serve with lingonberry jam, mashed potatoes, or boiled dill potatoes.