Preheat the oven to 175 degrees Celsius (350 degrees Fahrenheit)
Peel the celeriac and cut it into large chunks. Place the pieces into a food processor and pulse until everything is broken down into even pieces the size of rice. Place the rice in a bowl along with the shredded chicken
In a skillet on medium heat, cook the onion and garlic in olive oil for 3 to 4 minutes until the onion begins to soften. Add in the broccoli florets and about 1/4 cup water just to cover the bottom of the pan. Cover and steam the broccoli until it is just tender but not overcooked (approximately 5 minutes). Add the onions and broccoli into the bowl with the rice and chicken
Whisk together the egg and coconut cream before adding in the chicken stock and nutritional yeast. Pour the liquid into the bowl with the other ingredients and stir until well combined.
Transfer everything to a casserole dish and firmly press down to ensure its well packed in. Cover the casserole dish with tin foil and bake in the oven for 45 minutes. After 45 minutes remove the tin foil and bake for another 5 minutes to lightly brown the top. Serve warm