Mark Bittman shares ideas for making stock on the spur of the moment:

“Fortunately, there are almost certainly flavorful ingredients sitting in your fridge or pantry that can transform water into a good stock in a matter of minutes. The process may be as simple as simmering in water fresh herbs, mushrooms or even tea, or browning aromatics to create richness, or adding staples like crushed tomatoes or coconut milk. To further maximize flavor in minimal time, it pays to reach for ingredients that pack a punch, like miso, anchovies, chipotles, Parmesan rinds, sometimes even leftovers.”