Preheat oven to 500 F degrees.
Toss the cauliflower with 4 tablespoons of olive oil, season to taste with salt and pepper. Spread out in single layer on a baking sheet and roast until cauliflower is nicely charred around the edges, about 20 minutes total, flipping half way through.
In a pot, melt butter over medium heat. Add the shallot and garlic and cook until softened, about 5 minutes. Add roasted cauliflower, thyme, ½ cup of water and cream. Bring to a simmer then cover and cook 8-10 minutes. Remove from heat , season with salt and pepper and let cool.
Puree cauliflower mixture in a blender until smooth. Add egg 1 at a time; process until smooth.
Transfer cauliflower puree to the crust; smooth top with a spatula.
Toss radishes (or other veggie of choice such as beets or more cauliflower) with olive oil, season with salt and pepper. Arrange on top of puree, slightly overlapping in whichever shape you fancy.
Bake tart on a baking sheet until radishes are beginning to shrivel and filling is bubbling around edges, about 45 minutes.
Let tart cool slightly before serving. Enjoy!