Step 1Heat the pressure cooker insert. Add the butter and oil, stirring until the butter is melted. Add the onion and cook until softened but not browned. Stir in the garlic and cook until aromatic, about 1 minute.
Step 2Stir in rice, coating the grains with the fat and toasting for a minute or so.
Step 3Stir in the white wine and rosemary; the wine should quickly come to a simmer.
Step 4Add the pumpkin puree and 3 1/2 cups broth and stir to combine. Seal the pressure cooker and set the timer to cook at high pressure for 6 minutes.
Step 5Quick release the pressure, carefully uncover, and stir in additional broth if desired. Season with 1/2 teaspoon salt and a pinch of pepper. The rice should be almost al dente; if necessary, continue to simmer for 1 to 2 minutes until the rice is cooked and the liquid is thickened (timing will vary slightly depending on the pressure cooker).
Step 6When the rice is al dente, stir in the chopped walnuts and 2 tablespoons walnut oil. Stir in additional broth if desired to adjust the consistency.
Step 7Plate the risotto, topping each serving with a drizzle of walnut oil and a sprinkling of fresh grated Parmigiano-Reggiano.