Trim the bottoms of the Brussels sprouts. If you are working with large Brussels sprouts, cut them into quarters. For smaller ones, cut them in half. Rinse the Brussels sprouts once they are cut.
Snap off the ends of the asparagus. Rinse them under water.
In a small bowl, mix the fish sauce, soy sauce, brown sugar, potato starch and water. Set that aside.
Heat the oil in a wok or large sauté pan over high heat. When the pan is hot, add the garlic and chili peppers and stir them immediately. Let them cook for 15 to 20 seconds so that they season the oil.
Add the Brussels sprouts and cook them for 2 minutes, stirring frequently. If it looks like the Brussels sprouts are burning at the edges, add a splash of water (about 1 to 2 tablespoons) to the pan.
Add the asparagus and cook for another 2 minutes, stirring frequently. Again, if the vegetables are burning, add a splash of water.
Give the fish sauce mixture a quick stir and add the mixture to the wok. The sauce should thicken a little. Stir to distribute the sauce on the vegetables. Cook for another 1 to 2 minutes.
Turn off the heat and plate the vegetables. Serve with rice or your favorite grains!