Preheat the oven to 450°F and lightly brush an 8-inch cast iron skillet with olive oil.

Bring a large pot of salted water to a boil. Add the cauliflower and garlic and boil until the cauliflower is fork-tender, about 8 minutes. Use a slotted spoon to scoop the cauliflower and the garlic into a blender, reserving the pot of cooking water.

Set a bowl of ice water nearby. Add the spinach to the boiling water and use a slotted spoon to immediately remove it (it’ll wilt very quickly) and transfer to the ice water to cool completely. Drain and squeeze excess moisture from the spinach. Coarsely chop the spinach and add it to a large bowl with the artichokes and scallions.

Check to make sure that the cauliflower in the blender has (mostly) cooled, then add the yogurt, olive oil, mustard, and salt to the blender and blend until creamy. Add this mixture to the bowl with the spinach and artichokes and stir to combine.

Spread the spinach mixture into the skillet and top with the cheese and pinches of red pepper flakes. Bake for 15 to 18 minutes, or until the cheese on top is browned and bubbling.

Garnish with parsley, if desired, and serve with baguette slices.