In a 6 qt pressure cooker, select "Saute" and click on "high". Heat olive oil, add sausage and cook until browned, about 3-4 minutes. Crumble as it cooks.
Add mushrooms and shallots. Cook and stirring frequently until mushrooms are tender, about 5 minutes. Season with Kosher salt and ground pepper, to taste.
Stir in garlic and thyme until fragrant, about 1-2 minutes. Whisk in flour until lightly browned, about 1 minute. Add beef broth and Dijon mustard. Bring to a boil and then reduce the heat.
Add pasta shells. Stir well. Select "Manual" setting and set time for 5 minutes. Set spout to "Sealed".
After finished, do a Quick-Release, according to manufacturing instructions. Stir in sour cream and heat through, about 1-2 minutes. Stir in Parmesan until melted, about 1 minute. Season with Kosher salt and ground pepper, to taste.
Serve immediately. Garnish with parsley, if desired.