Cut the cauliflower into even florets so they cook at the same rate, then set them in a steamer basket over a pot of boiling water. Cover and steam for 12 to 15 minutes, until a knife slides into the thickest stem with no resistance.

While the cauliflower steams, combine the olive oil and sliced garlic in a small skillet over medium-low heat. Cook for 3 to 4 minutes, stirring often, until the garlic turns soft and pale gold. Pull it off the heat before any of it browns, because burnt garlic turns the whole mash bitter.

Lift the steamed cauliflower into a dry skillet in a single layer and set it over medium heat. Toss it for 3 to 4 minutes, until the surface looks matte and you stop seeing steam rise. This step is what keeps the mash thick instead of watery.

Add half the cauliflower to a food processor with half the garlic oil, vegan butter, oat milk, nutritional yeast, salt, and pepper. Pulse until mostly smooth, stopping to scrape down the sides, then repeat with the second half.

Taste and adjust the salt, adding more oat milk a tablespoon at a time only if the mash feels tight. Serve warm with a spoon of vegan butter and a scatter of chives or parsley.