Place the oil and onion into a medium stock pot, then add thyme and garlic salt; saute them for 2 to 3 minutes.
Add 10 cups of water and bring to a boil, then add in the green and yellow split peas and barley. The water should be about two inches above the top of the immersed peas; add more liquid if necessary.
Add the vegetable stock cubes, bay leaves, and simmer at a gentle boil until the split peas
start to break up, about 30 minutes (stir with a spoon or whisk the peas to help break them up; this gives the soup its wonderful thickness).
Next, add in the carrots and celery and season with garlic salt and pepper to taste. If the soup is too thick, add more water.
Just before serving, add in the parsley and remove bay leaves. This pot of soup feeds a crowd as a side dish and makes a hearty, healthy meal along with a green salad and crusty bread for a group of 8.