In a large pot over medium-high heat, add a drizzle of olive oil, then place in half the mushrooms. Leave to brown on one side, flip them over, then season with a pinch of salt and pepper. Continue frying until nicely browned, then remove to a side bowl and repeat with the second batch, adding more oil as needed. Remove again once done. It's important not to cram in the mushrooms, or they'll steam and lose flavour. Also important to not move them about too much, or they won't brown.
Melt 1 tbsp butter, then add the onion. Fry until it begins to brown/soften, then add in the garlic. Continue frying for another 30 seconds or so, then pour in white wine. Allow the wine to simmer and reduce for a good 5mins, until it's almost all evaporated, deglazing the pan as necessary. It's important to ensure you reduce the wine or it'll be too strong in the sauce.
Pour in the orzo and add back in the mushrooms. Pour in stock and cream, then sprinkle in parsley and thyme. Give it a good stir, then bring it to a simmer. Lower the heat slightly, then continue stirring somewhat frequently until the orzo is al dente and the sauce has thickened.
Remove from heat and stir through the parmesan and 2nd tbsp of butter. If you over reduce the sauce and it looks a little dry, just add a splash more hot stock or water. Season to taste (generously if needed), then serve with extra parmesan/parsley!