Wash and peel the celeriac, removing any brown veins. Cut into approximately 1-inch cubes.
Parboil celeriac cubes for about 10 minutes in a large pot filled with the lemon juice and enough water to cover. Remove celeriac and reserve 2 ½ cups of water.
Heat olive oil and butter in a large pot and sauté onions and carrots until tender but not fully cooked. Add partially cooked celeriac and peas along with the 2 ½ cups of hot water, salt and pepper. Simmer until vegetables are soft but not falling apart, about 15 minutes.
In a bowl, mix egg yolks and lemon juice. Slowly add about a cup of hot liquid from the pot, stirring continuously so the egg yolks are incorporated without coagulating. Pour the egg mixture into the pot and mix thoroughly.
Sprinkle with dill and serve immediately.