Using a sharp knife, peel the celery root. Discard peelings, rinse well and pat dry.
Chop celery root into smallish chunks, place in a food processor, and blitz until reduced to small, rice-sized pieces - you should have roughly 2 cups worth. Set aside.
In a large pan, heat olive oil. Add onion, saute until onion softens. Add riced celery root, continue cooking until heated through. Add dill, mint, and lemon juice, stir well and season with salt, to taste. Remove from heat and set aside.
Carefully take the grape leaves out of the jar, rinse well, and drain. Trim all stems off the leaves, and get ready to roll!
Place a large leaf in front of you, shiny side down and with the bottom facing you. Add a bit of filling - usually between 2-4 tsp worth, or about a tablespoon, plus or minus a little. Fold the bottom of the leaf up and over the filling, tucking everything tightly as you do so. Fold the sized in, and roll it up like a burrito. It may be awkward at first, but you’ll find your groove after a few!
Roll up all of the filling into the leaves, placing each finished roll into a medium/large pot - seam sides down, and tightly packed. If they don’t fill the bottom of the pot, consider moving them to a smaller one - loose packing will make them come undone as they cook.
Whisk together lemon juice and olive oil, drizzle over the rolls. Pour chicken broth over the rolls. Gently simmer - not boil - over low heat for about 15 minutes. Serve warm with toum.
If using dried mint and/or dill, start with 2 Tbsp of each, and adjust to taste.