While the cauliflower is baking, make your sauce so it's ready to go when that cauliflower is fresh out of the oven! First, heat a sauce pan on medium heat.
Add 3 tbsp of olive oil to the sauce pan, then add the garlic, onion and tarragon to the oil. Let that simmer on medium low heat for 10 minutes. You do not want the oil to be too hot, as it will burn the aromatics. We just want to infuse the oil with all of that flavor.
Add 2 tsp of flour to the olive oil to create a roux. Mix for about 1 minute, until the flour is lightly toasted.
In a separate cup, combine the almond milk, vegetable broth and lemon juice (if using.) Iif not using vegetable broth or lemon juice, of course just get that almond milk ready.
Adjust the heat to medium high. Slowly pour the liquid mixture into the sauce pan, whisking as you go. Continue to whisk until the sauce is fully incorporated. Season generously with salt (I did about 2 generous pinches) and taste to make sure you like the salt levels.
Let the sauce begin to bubble and reduce until thick, whisking every 30 seconds or so.
When the sauce is your desired thickness, remove the sauce pan from the heat and set it aside.