Cut cauliflower or broccoli into small florets of uniform size (about 12–15 pieces total); reserve other parts for other uses. Cut the carrots, leek, and zucchini into very small cubes (brunoise) or narrow sticks (julienne), keeping each vegetable separate.
Blanch the cauliflower florets for 3 minutes (or broccoli florets for 1.5 minutes) in boiling salted water, then drain. Blanch the snow peas for 2 minutes in the same boiling water, then drain.
In separate pans over medium heat, sauté the zucchini, carrots, and leek in butter for roughly 3–4 minutes each until tender. In another pan, finish cooking the blanched cauliflower and snow peas in butter for about 2 minutes until heated through. Season all vegetables with salt and pepper.
Reduce the velouté sauce by simmering over medium heat for roughly 5 minutes until noticeably thicker. Line a baking sheet with parchment paper and place a setting ring (8–10 cm diameter, at least 4 cm high) on the sheet. Layer the cooked vegetables in the ring up to the edge, pressing firmly. Repeat to create four stacks. Before removing the ring, pour roughly 50 ml of the cooled, thickened velouté over the vegetables to stabilize them, then carefully lift off the ring.
Preheat the oven to 220 °C (425 °F). Use the setting ring to cut four round slices from the puff pastry. Pierce each slice 10–15 times with a sharp knife. Place one pastry slice on top of each vegetable stack. If desired, weight down the pastry with an ovenproof saucer to prevent excessive rising. Bake for 12 minutes until golden.
While the pastry bakes, warm the Bercy sauce over low heat without boiling. In the last 4–5 minutes of baking, melt the remaining butter in a pan over medium-high heat and sauté the scallops, turning occasionally, until opaque and cooked through. Season with salt and pepper.
Spread 2 tablespoons of Bercy sauce in a decorative pattern on each of 4 serving plates. Carefully transfer one millefeuille to each plate using a cake server. Drizzle 1 tablespoon of Bercy sauce over the puff pastry, place one scallop on top, and serve immediately.
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