Melt 2 tablespoons of the butter and olive oil in a high-sided saucepan. Cook the shallots and celery, about 5 to 7 minutes. Stir in the rice and cook for another minute or so.
Slowly ladle in the heated stock (one or two ladlefuls at a time), stirring until fully absorbed by the rice between each addition. Keep stirring until the rice is perfectly al dente (to the tooth), about 18 to 20 minutes.
Stir in the lemon zest and rosemary.
In a small bowl, whisk together the egg yolk, juice of half the lemon (plus more to taste), Parmesan, cream, and salt and pepper. Stir this mixture into the finished risotto, along with the remaining 2 tablespoons butter. Cover the pan and let sit for 2 minutes or so, until finished cooking.
Vigorously stir, plate, and garnish with more Parmesan.