Preheat the oven to 450°. Remove the tender leaves from the broccoli stems; wash leaves and set asidel
Separate the florets from the large stems. Leaving some stem attached, cut enough florets into 1" pieces to measure 4 cups (set aside remaining florets for another use).
Cut the bottom 1" from the stems and discard. Using a vegetable peeler, remove the woody outer layer to expose the pale-green center (you'll need to remove 2-3 layers). Quarter the stems lengthwise. With the tip of a paring knife, make shallow crosshatches into the flat sides, if desired (this increases surface area, allowing broccoli to absorb more flavor).
Place florets on a rimmed baking sheet; drizzle with 2 tablespoons grapeseed oil and toss to coat. Season with salt and pepper. Roast the florets, turning once, until charred, 15 to 20 minutes. Let cool; mince florets.
Whisk shallots, vinegar and lemon zest in a medium bowl; Gradually whisk in olive oil, then minced florets. Season vinaigrette to taste with salt and pepper.
Toss broccoli stems and remaining 1 tablespoon grapeseed oil in a large bowl, Heat a large skillet over medium-high heat. Working in batches, sear stems on all sides until browned in spots and crisp-tender, 3-4 minutes per batch. Divide among plates. Drizzle with vinaigrette and garnish with reserved broccoli leaves.