Finely dice the onion and celery and crush the garlic. Heat the butter in a skillet over a low/medium heat. Add the onion and celery. Sweat gently for a few minutes until tender and fragrant.
Add the cauliflower rice, sage, ground almonds, orange juice, and zest to the skillet. Stir well to combine. Pan fry gently for 3-4 minutes to soften the cauliflower and toast the almonds.
Roughly chop the cranberries and walnuts. Add to the skillet and stir well to combine. Pan fry for a few minutes until the walnuts are lightly toasted.
Preheat the oven to 375 degrees Fahrenheit and lightly oil a shallow oven tray. Wipe the mushrooms clean and remove the stalks. Use a teaspoon to scoop the gills from the mushrooms and discard. Arrange the mushrooms across the prepared oven tray.
Divide the stuffing evenly between the mushrooms, filling the centers of each. Transfer to the oven and bake for 15 minutes or until the mushrooms are cooked through and tender, and the filling is lightly golden on top.