In the pressure cooker, on saute setting, melt butter. Add onion, garlic and ¼ tsp salt. Saute for 3 minutes. Stir in portabella mushrooms, cremini mushrooms and remaining ¼ tsp salt. Cook for 5 minutes, stirring frequently. They will begin to release liquid and shrink to about half the volume.
Stir in white wine and cook another 2 minutes or until alcohol smell has cooked off.
Add dried shiitake mushrooms, chicken broth and thyme.
Lock lid and point valve to sealing. Set to cook at high pressure for 5 minutes. When the cook time is complete, carefully release pressure.
Remove sprigs of thyme and blend until soup has a smooth consistency. Stir in half and half.