Preheat oven to 375 degrees. Clean all vegetables and trim, cut and wedge to desired shapes (eighths are best for celeriac and onions, quarters for fennel and turnips; and simply split beets across their equator to highlight their natural, internal rings). Reserve fennel fronds for garnish.
Place all vegetables in a large bowl. Combine syrup, lemon juice, salt, pepper, sage and olive oil. Reserve half the mixture and drizzle the other half over vegetables.
Spread vegetables onto parchment-lined sheet pans in a single layer, pushing large pieces to outer edges of pan and leaving room between each vegetable (if they are touching or too close, they will not brown and caramelize, but rather steam and soften).
Roast 30 min. Remove from oven, drizzle with 1/2 of remaining glaze, reserve remainder and return to oven. Roast 10 min more or until vegetables have browned and are soft in center when pierced.
Remove from oven, sprinkle with roasted garlic cloves and glazed pecans, and return to oven just until heated through, approx. 3 min. Garnish with more pecans, fennel fronds and/or sage, as desired, and serve immediately with remaining glaze on the side.