Preheat oven to 350 degrees. Place the mushrooms on a baking sheet and toss with 2 tablespoons oil. Season with salt and black pepper. Place on a rack near the bottom of the oven and roast while starting the risotto.
Heat remaining 2 tablespoons oil and butter in a heavy ovenproof skillet or Dutch oven over medium-high heat.
Add onion, reduce heat to medium and cook, stirring often until softened and slightly translucent, approximately 4-5 minutes.
Add the garlic and cook briefly 10-15 seconds or until fragrant.
Stir in the rice. Cook stirring occasionally until many of the grains are translucent, 2-3 minutes.
Add the wine, bring to a simmer and cook stirring until wine has almost evaporated.
Add 2 1/2 cups of the broth. Bring to a simmer, cover and place in the oven. Bake for 16-18 minutes.
Return pot to the stove on low heat along with taking the mushrooms out of the oven.
Add remaining 1/2 cup stock, heavy cream, Parmesan cheese and pesto. Stir until smooth.
Sprinkle with chopped walnuts and garnish with chopped parsley. Serve with additional Parmesan cheese at the table.