In a saucepan, bring the stock to a simmer; cover and keep warm
In a medium dutch oven, melt the butter and oil. Add onion and cook over moderately high heat, stirring, until softened (about 5 minutes). Add the shredded beets and cook, stirring, until the pan is dry, about 12 minutes. Spoon half the beet mixture into a small bowl
Add the rice to the dutch oven and cook, stirring, for 2 minutes. Add 1 cup of the warm stock to the rice and cook over moderate heat, stirring, until stock is nearly absorbed. Continue adding stock one cup at a time, stirring constantly, until the rice is al dente and a thick sauce forms, about 25 minutes
Stir in the cooked beets, cheese, and poppy seeds. Cook, stirring, until heated through. Add a few tablespoons of water if the risotto is too thick. Spoon the risotto into bowls and garnish with sliced beets and poppy seeds