Using the saute feature of the Instant Pot melt 1 Tbsp. butter and add in the chopped onion and diced garlic. Cook for about 5 minutes or until onion is translucent.
Add in 1/4 cup butter and melt, whisk in flour and cook for a few minutes or until smooth and bubbly.
Turn off heat and whisk in the chicken broth.
Add in carrots, broccoli, salt, paprika, mustard, pepper, and nutmeg.
Secure lid and cook on high for 8 minutes.
Allow to naturally release for 10 minutes , and then fully release.
Add in shredded cheese and half and half, stir until well incorporated or for a smoother texture blend with an immersion blender.