Put the onions in a steamer basket over about 3 in (7.5 cm) of boiling water; steam until completely tender, 12 to 15 minutes. While the onions are steaming, toast the coriander seeds and Arborio rice in a small, dry pan(without oil) over medium heat until fragrant and lightly browned, 4 to 5 minutes.

Spoon the soft, steamed onions into a medium saucepan over medium heat and pour almond milk over the onions to cover. Add the toasted coriander and Arborio rice and simmer until reduced by a quarter, about 15 minutes.

Let cool until just warm, then transfer the mixture to a blender and purée for 3 to 4 minutes, scraping down the sides of the blender several times to make sure everything is fully incorporated. Use this immediately as the base for a creamy soup, or store in an airtight container in the refrigerator for up to 1 week. You also can pour the soubise into an ice cube tray and take out cubes of it as needed. Frozen soubise cubes will last in the freezer for up to 3 months.