Dump the split peas into a bowl, cover them with warm water and let them soak at least four and up to eight hours. Drain, and rinse well.

Toss the peas into a medium pot, cover with stock. Bring them to a boil over high heat, then turn down the heat to medium and simmer, covered, until the peas are tender and cooked through. Turn off the heat, replace the lid to keep them warm.

Heat the oven to 425 F.

Toss the carrots, parsnips and celeriac in a large bowl with olive oil, stirring them until they're evenly coated. Spread them on a rimmed baking sheet, and season them generously with salt. Roast them for 20 minutes or until cooked through, with their edges beginning to caramelize.

While the vegetables roast, fry the bacon in a cast iron skillet over medium heat until crisp. Using a slotted spoon, transfer the bacon to a waiting plate. Stir the chopped onion and fresh thyme into the hot bacon fat and cook until fragrant and translucent, about 6 minutes.

Transfer the onion and bacon fat, reserved split peas and white pepper to a food processor. Process until smooth. Generously adjust the seasoning of the puree with salt, and process about 30 seconds longer.

Spoon the purée into a bowl, top with roasted vegetables and reserved bacon. Serve warm with buttered rye bread and ham.