Preheat oven to 400 degrees F (200 degrees C). Coat a shallow baking dish with cooking spray; set aside.
Add chicken stock to a large saucepan; bring to a roiling boil over medium-high heat. Stir in polenta and salt, reduce the heat, and simmer, stirring frequently with a long-handled spoon until soft and thickened, about 8 minutes.
Remove from heat and stir in pesto; stir in 1 cup shredded mozzarella. Pour into the prepared pan, then smooth the top with the back of a spoon.
Bake in the preheated oven for 20 minutes. Remove and check for firmness. If it is still too soft in the middle, let it bake 10 minutes more.
Remove polenta from the oven; spread marinara over the top, then sprinkle remaining mozzarella cheese on top. Return to the oven and bake until cheese is golden, about 15 minutes more.
Let cool for 10 minutes. Slice and serve.
