Set Instant Pot to Sauté. Warm olive oil with 1 tablespoon butter until shimmering.
Sauté mushrooms until browned and their liquid evaporates, 5–7 minutes. Transfer to a bowl.
Melt remaining butter in the pot. Add onion and garlic; cook until translucent, about 3 minutes.
Stir in rice and thyme. Toast, stirring, until edges look pearly, 1–2 minutes.
Pour in wine and scrape up browned bits. Simmer until mostly absorbed, about 1 minute.
Add broth and 1 teaspoon salt. Cancel Sauté, lock lid, and cook on High Pressure for 6 minutes.
Quick-release pressure. Open lid and stir briskly to release starch and thicken.
Fold in mushrooms, Parmesan, black pepper, parsley, and lemon juice. Stir in cream if using. Rest 2 minutes and serve.