Turn your oven to 400 degrees. In an oven-safe skillet or roasting pan, toss the whole garlic gloves and chopped onions in some oil, salt and pepper. Roast for 30-35 minutes, stirring occasionally, until browned and and soft. Meanwhile, in a small pot, heat up the stock till it's simmering.
Scrape the onions and garlic into a heavy-bottomed soup pot (not the pot you're using to warm the stock) and add in the mustard seeds. Cook over medium-high heat for 2 minutes, and then add in turmeric. Add in spinach and a generous pinch of salt. Stir to wilt spinach, and add in about 2/3rds of the stock. Allow it all to cook down for 7-10 minutes.
Using an immersion blender (or pour into an upright blender), begin to blend the soup, adding more stock to thin it if necessary. It doesn't need to be totally smooth. Drizzle with a splash of heavy cream or almond milk, and enjoy!