Turn on the Sauté setting on the Instant Pot. Sauté 2 oz onion, 1 tbsp garlic, and 2 oz carrots in 1 tbsp olive oil and 1/2 tbsp unsalted butter for 2 minutes. Stir occasionally until the onion starts to turn translucent.
Add 8 oz mushroom. Sauté for 3 minutes.
Add 6 oz tuna, 4 oz green peas, 1/2 tsp salt, 1/2 tsp thyme, 1/2 tsp ground black pepper, 1/4 tsp paprika, 2 tsp Dijon mustard, and 1 tsp lemon juice. Stir.
Add 6 oz tagliatelle pasta, 1/4 cup whole milk, and 1.5 cup unsalted chicken broth. Stir.
Close the lid and set the steam release valve to the sealing position. Cook on the “Meat/Stew” setting for 10 minutes.
When the timer is up, open the lid, and turn the steam release valve to the venting position to perform a quick-release of the steam. Stir 4 tbsps heavy cream and 2 oz cheddar cheese into the Instant Pot.
Take the pot out. Divide the tuna casserole into serving portions and sprinkle 2 tbsp freshly chopped parsley on top. Serve hot with a side dish of choice.