The magic ingredient was a small packet of dried porcini mushrooms. Soaked them in hot water for half an hour prior to cooking, as per the instructions on the packet, after which time they were rehydrated and could sensibly be shoved in the pan along with the rest of the (ordinary) mushrooms.

However, a lot of mushroomy goodness had diffused out into the water I soaked them in, which had become nearly opaque in its sheer mushroomness. So I reused that water during cooking to produce an extremely mushroomy result. (Since I cheated and used a stock cube, that was just a matter of dissolving the cube in the mushroomy-water rather than in any old water; if I'd been using proper liquid stock then I suppose I'd have had to do something more cunning, such as carefully not using too much water to soak the mushrooms and then mixing half-and-half with the stock.)

Other than that, I think, the rest was just conventional risotto technique.

I do sometimes use dried mushrooms but I tend to just throw them in and hope that the stock from the cooking will rehydrate them. Perhaps I should actually try rehydrating them properly. I want to go and make risotto now!