If using frozen tempeh, make sure it’s thawed at room temperature. See the notes below on other tips to prepare tempeh.
Slice the tempeh into 1/2 to 1” thick pieces.
If using tofu, press the tofu for 10 tp 15 minutes then slice it into 1" thich pieces.
Heat a large non-stick pan over medium high heat. Once hot, add some neutral oil to coat the surface of the pan. Pan-fry the tempeh/tofu until golden brown, around 3 minutes on each side.
Remove the tempeh/tofu from the pan and set aside.
While the tempeh is pan-frying, mix all the sauce ingredients together in a small bowl. Taste the sauce and feel free to adjust to your desired taste. Note that if using store-bought kecap manis, it can be watered down so I recommend adding more if needed.In another small bowl, mix together the cornstarch and water to create a slurry. Set aside.
In the same pan used to pan-fry the tempeh, sauté the chili and onion over high heat in the oil for 2 minutes.
Pour in the sauce and leave to simmer over medium high heat.
Once it boils, lower the heat to medium.
Give the cornstarch slurry a good mix again. While mixing, pour in the cornstarch slurry into the sauce. Lower the heat to low.
Keep mixing until the slurry is well incorporated in the sauce.
Add back in the pan-fried tempeh and leave to simmer in the sauce for 4 to 5 minutes. Turn up the heat to medium and then flip over the tempeh to coat evenly in the sauce. The sauce will start to thicken and turn into a glaze.
Once the sauce thickens, lower the heat to prevent the sauce from burning.
Cook for 2 to 3 more minutes and flip the tempeh to evenly coat in the sauce. The sauce will thicken into a glaze-like consistency. Taste the tempeh and feel free to add more kecap manis or season with some salt, to taste.
Turn off the heat then remove the tempeh from the pan.