Pick off the large cauliflower leaves. Turn the cauliflower upside down on a cutting board. Using a paring knife, carefully cut out about two inches of the stalk and core. Your cauliflower should have a cavity for the stuffing without breaking apart the florets. You should be left a hole for the stuffing without breaking apart the florets.
Pre-cook your cauliflower by bringing about two inches of water to a boil (with the lid on), add the salt and stir. Carefully add the head of cauliflower to the water, bring water to a simmer, place the lid over the pot and steam the cauliflower for 6 minutes, until just fork-tender. Very carefully lift the cauliflower out of the pot and set it aside to cool.
While the cauliflower cools, make the stuffing. To a saute pan, add the olive oil, carrot, celery, and onion, and cook until the onion softens and begins to become translucent. Add a half tablespoon of olive oil, then add the garlic. Cook until fragrant, about 30 seconds. Mix into the carrot mixture, remove from heat and set aside.
To a medium-sized bowl, add the egg, mayonnaise, and mustard. Whisk until combined and clump-free. Add the spices, herbs, lemon zest, and juice whisk until combined. Slowly add carrot mixture to egg mixture, whisking in between to temper the egg mixture. Add the bread crumbs, and mix thoroughly.
You can use a piping bag to stuff the carrot mixture t into the crevices of the cauliflower or gently use a spoon and your hands. Once the stuffing has been placed inside the cauliflower, carefully turn it over. Spread the mayonnaise over the top and sprinkle with paprika.
Bake at 375 in a preheated oven for 30 minutes. Cool for 5 minutes, then slice the cauliflower into steaks or quarter pieces. Top with garlic dill butter before serving.