- For the cauliflower purée, place all ingredients in a medium pot, making sure to add enough water so cauliflower is just covered by the liquid. Bring to a boil and then lower to a simmer. Simmer for 45 minutes.
- While purée is simmering, prepare fish. Heat canola oil in a large cast iron or heavy pan. Pat fish dry and season with salt and white pepper. Sear fish on one side.
- Lower to medium heat. Turn fish over and add the butter and thyme to the pan. Gently spoon the butter over the fish until fish is cooked through, about 3−5 minutes. Remove and keep warm until rest of dish is ready.
- For mustard greens, add oil to same pan and bring to high heat. Wilt the greens slightly and add shallot and butter. Season with salt and remove from the pan.
- For roasted cauliflower, add oil to same pan and bring to high heat. Add cauliflower and lower to medium heat. Caramelize both sides and remove to a paper towel-lined dish. Season with salt.
- For the vinaigrette, combine all ingredients, except pepper, lemon, and oil, in a bowl. Allow to macerate for 20 minutes. Whisk in the oil slowly and crack the pepper directly into the vinaigrette.
- Place the cooked cauliflower purée that’s been simmering in a high-powered blender and blend with just enough of the liquid to form a smooth purée, about ¼ of the amount in the pot, discarding the rest. Adjust seasoning.
- To plate, place a little of the purée on four plates. Arrange the roasted cauliflower, mustard greens, and lemon fish. Spoon vinaigrette over the fish and vegetables and serve.