1. Prepare vegetables. Leek; remove stem, and all of the dark green part. Cut lengthwise almost all of the way through and rinse between the layers to remove any sand, then thin slice crosswise. Garlic; mince fine. Cabbage; cut into thin strips and chop into 1 to 2 pieces.
2. Dissolve bouillon in 1C boiling water and set aside.
3. Over medium heat, melt butter and oil in small dutch oven (or whatever covered pan that will work), add all vegetables. Cook until translucent and soft (5 minutes or so), stirring often and adding 1T of reserved veg bouillon at a time (if needed) to keep vegetables from burning.
4. As vegetables near doneness, boil the remaining 1&1/2C water. All at once add boiling water and remaining bouillon, heat to boil. Turn off burner and add package of instant mashed potatoes and any salt/pepper to taste. Stir well, cover and let sit for 5 minutes. Remove cover, stir well to blend and serve hot.
5. There are many possibilities for what to serve with this.
6. Suggestions: tossed salad, sourdough bread and butter, top with grated cheese of your choice or sour cream. I've seen many Colcannon recipes that call for diced ham, etc. We love it plain.