Heat stock in a pan and keep hot.
In a small bowl, reconstitute porcini in a little hot stock; set aside.
In a large sauté pan, melt 2 tbsp butter and olive oil over medium heat.
Sauté shallots and celery until soft. Salt them.
Add mushrooms to pan. Drain the porcini, chop finely and add to mushroom mixture, reserving the liquid.
Sauté for a few minutes, then add arborio rice and continue to cook, stirring, until kernels are coated with oil and smelling a little toasty.
Add wine combined with the strained liquid from the porcini and cook, stirring, until almost completely absorbed. Add thyme.
Begin adding ladles-full of stock, stirring often, only adding more as the liquid incorporates. After 15 minutes or so, taste the rice.
When it is cooked but still al dente, add remaining 2 tbsp of butter and Pecorino Romano cheese.
If a creamier consistency is desired, stir in a little more stock. Conversely, if the rice is too soupy, cook a little longer. The whole process should take about 25 minutes. Serve immediately.
