Preheat oven to 450° with racks set in upper and lower thirds. Prepare cauliflower: Trim leaves and stem (to base of florets). In a medium bowl, whisk together harissa paste, the oils, lemon juice, and 1 tsp. salt. Holding cauliflower over sauce, spoon sauce all over cauliflower, including underneath. Pour remaining sauce into a small serving bowl and top with chile flakes.
Set cauliflower on a parchment-lined rimmed baking sheet and sprinkle lightly with salt. Roast until very tender when pierced with a knife, about 1 hour.
Meanwhile, prepare kabocha squash: Rinse squash and pat dry, then microwave 5 minutes to soften. With a short, sharp knife, carefully cut around squash stem to make a 4-in. lid. Pry out lid with a table knife (you may have to cut a sliver out of lid to get the knife in), then scrape out and discard seeds.
In a bowl, whisk together miso, tahini, lemon juice, soy sauce, and 2 tbsp. water. Brush insides of squash and lid with some of miso tahini glaze, then pour rest of glaze into a serving bowl; thin with another 1 to 2 tbsp. water. Sprinkle sauce with sesame seeds and set aside.
Rub squash all over with oil and sprinkle with salt. Set lid ajar on squash. Remove baking sheet with cauliflower from oven. Set squash next to cauliflower on sheet and roast until tender when pierced with tip of a knife, 30 to 45 minutes.
Prepare cabbage: Set cabbage on its stem end and cut into quarters halfway down to stem. Insert a toothpick into each quarter from the outside to hold leaves together. Microwave cabbage until tender when pierced with tip of a knife, 5 to 7 minutes.
Meanwhile, in a small bowl, whisk together caraway seeds, 1/4 tsp. salt, the Dijon mustard, lemon zest and juice, and oil.
Set cabbage, cut side up, on a second rimmed baking sheet. Drizzle all over and inside with vinaigrette.
Roast cabbage in same oven as cauliflower and squash until well browned, 15 to 20 minutes.
To serve: Set all three vegetables together on a carving board. (The skin of a kabocha squash is entirely edible, so carve it into slices to serve.) Set reserved harissa sauce next to cauliflower, miso tahini glaze next to squash, and Parsley Ginger Horseradish Sauce next to cabbage.
Make ahead: All 3 vegetables, up to 2 days, chilled; reheat in microwave until hot. Sauces, up to 2 days, chilled.
Drink pairings. Squash: A rich, fruity white like Calera 2014 Viognier (Mt. Harlan; $32). Cabbage: A crisp, aromatic such as Imagery Estate 2015 Upper Ridge Riesling (Pine Mountain/Cloverdale Peak; $26). Cauliflower: Firestone Walker "Opal" Farmhouse Ale, Dry Hopped Saison (22 oz.; $8).
Note: Nutritional analysis is per serving with 2 tbsp. of each sauce.