Set your oven to broil on high. Trim the leaves and about 1 inch of stem off the cauliflower, leaving the whole head intact. Slice the cauliflower, from the top down, into 1 inch thick steaks. On a baking sheet, gently coat both steaks in 1 tbsp olive oil and season with salt and pepper. Place on the middle rack and broil until well-browned, about 18 to 24 minutes.
Peel and dice the parsnips. Add them to a small saucepan, cover with 1 inch water, and add a pinch of salt. Bring parsnips to a boil and cook until fork tender, about 12 to 14 minutes. Drain and return to the saucepan.
Trim about 1 inch from the bottom of the asparagus. Once the cauliflower has been roasting for 15 minutes, add the asparagus to one side of the baking sheet and toss with 1 tsp olive oil. Sprinkle with salt and pepper and roast until bright green and just tender, about 6 to 8 minutes. At this point, the cauliflower should be well-browned.
With the back of a fork, mash the cooked parsnips until smooth. Add the Vegenaise, French mustard herb blend, 1 tbsp olive oil, and a pinch of salt and stir to combine. Remove from heat and cover to keep warm. Place a medium skillet over medium heat and add the pumpkin seeds. Cook, shaking the pan frequently, until toasted, about 3 to 5 minutes. Transfer the toasted pumpkin seeds to a small bowl.
Peel and mince the shallot. Return the skillet to medium heat with 1 tsp olive oil. Add the minced shallot and cook until slightly softened, about 2 to 3 minutes. Add the butter and capers, and stir until the butter has melted, about 1 minute. Zest the lemon, then halve it. Add the juice from half the lemon and a pinch of salt, and stir to combine. Bring to a simmer and then remove from the heat.
Cut the remaining lemon half into wedges. To serve, spread the parsnip mash onto large plates and top with the cauliflower steaks and roasted asparagus. Drizzle with lemon caper butter and sprinkle with toasted pumpkin seeds and lemon zest. Serve with lemon wedges.