Warm a small dry skillet over medium heat. Add the raw pecan pieces and toast, stirring often, until fragrant and light golden brown, about 3 to 5 minutes. Immediately transfer the nuts to a plate so they do not continue cooking, then let cool completely.

Cut the base off the Brussels sprouts and pull away any wilted or damaged outer leaves. Shred the sprouts with a food processor fitted with the slicing disc, or use a mandoline set to 1/8 inch for very thin ribbons. A sharp chef's knife also works; just keep the slices as even as possible.

In a large bowl, toss the shredded sprouts with the green onion, apple matchsticks, and rosemary. Pour the dressing over the top and toss until every piece is coated. Serve right away, or let it rest for 10 minutes so the sprouts soften and absorb the dressing.

In a small bowl or liquid measuring cup, whisk together the Meyer lemon juice, garlic, agave nectar, Dijon mustard, kosher salt, and black pepper until the salt dissolves.

With the whisk moving constantly, pour the olive oil in a slow, steady stream. Keep whisking until the dressing looks smooth and lightly emulsified. Use immediately or store in the fridge.