Heat the oil in a saucepan or wok over medium-high heat.
Add the mustard seeds and when they sputter, turn down the heat to medium and add the potatoes and garlic.
Saute the potatoes, stirring occasionally, until they begin to take on a golden hue.
Add the chopped kale, stir until it starts to wilt. Add the soy sauce and hot sauce, stir well, and add a couple of tablespoons of water. Slap on a tight-fitting lid and let the kale steam. If the water evaporates, add a couple more tablespoons and repeat until the kale is quite tender, about 30 minutes.
Stir in the lemon juice and salt, if required. Turn off the heat and serve hot.