Chop the cabbage and add to a large bowl. Add salt and massage it into the cabbage until it starts to soften. Add water to cover the cabbage. Stir a bit to dissolve salt into the water. Place a heavy plate over it to weigh the cabbage down. Let it sit for a minimum of 6 hours or overnight.
The next morning, drain the cabbage and save the brine. Add it into a bowl along with chopped onion, scallion and grated carrot.
Make a spice paste by mixing minced ginger, garlic, soy sauce, sugar, gochugaru and a little water (if needed).
Using your hands, gently mix this spice paste with the vegetable. Mix well until well coated.
Pack this kimchi in a quart jar and weigh it down using a wooden spoon. Pour the brine you saved on top to cover the vegetables. Leave an inch space at the top. Cover loosely by placing a small zip lock filled with brine that serves the purpose of weighing it down.
Let it sit to ferment in cooler room temperature away front direct sunlight for 3-6 days.
Check every day once. Weigh it down a bit. Check if the vegetable is covered in brine. If not top with more brine. This process also burps it as it lets the air out of the jar.
On day three, check if the flavor is sour enough for you. For tangier kimchi, keep fermenting until day six. Just keep checking it every day and refrigerate if need be.
Refrigerate it after the sixth day. It stays good in a refrigerator for a year. The flavors keep getting complex and tangy as time goes.