Preheat oven to 500. Arrange chopped cauliflower in a single layer on a sheet pan that has been lightly sprayed with nonstick spray. Season with salt and pepper. Roast for 25-30 minutes, stirring every 7-10 minutes until tender and beginning to brown deeply.
Arrange zucchini slices in a single layer in a sheet pan that has been lightly sprayed with nonstick spray. Season with salt and pepper. Roast 10 minutes, flipping once after 5 minutes.
Allow veggies to cool slightly, then add to a blender or food processor with the remaining ingredients. Blend until smooth, adding small amounts of water as needed to reach the desired consistency. You should not need more than a couple tablespoons. Taste and adjust seasoning with salt, pepper, or additional lemon juice or cayenne.