This creamy salad highlights celeriac (celery root), a knobby, brown root whose crisp flesh tastes like a cross between celery and parsley. To keep it from browning, peel and grate just before adding to the dressing.
Preheat oven to 400°.
Remove white papery skin from garlic heads (do not peel or separate the cloves). Cut 1/4 inch from the top of each head; lightly coat cut surfaces with cooking spray; sprinkle with 1/4 teaspoon salt and 1/8 teaspoon pepper. Wrap each head separately in foil. Bake at 400° for 45 minutes; let cool 10 minutes. Separate cloves; squeeze out garlic pulp; discard skins.
Place garlic pulp, 1/4 teaspoon salt, 1/8 teaspoon pepper, sour cream, broth, vinegar, mustard, and sugar in a blender; process until smooth. Pour dressing into a bowl; cover and chill 2 hours.
Combine celeriac and chives in a large bowl; add dressing, and toss well. Serve immediately.