There is no true substitute for dashi. But I did some experimentation and found something that comes pretty close. The magic of dashi is the iodine in kombu set against umami, usually in the form of shiitake, dried sardines, or katsuobushi. Bacon makes a decent approximation to katsuobushi. And, as it turns out, potatoes are high in iodine, and play a solid stand-in for kombu.
What you will need
2 qts (2L) water
1/2 lb (225 g) potatoes
1 lb (500g) bacon
How to cook
Place the water in a medium stock pot. Bring it just barely to a simmer over medium heat.
Drop in the bacon. I used uncured bacon. Still a great flavor, but without the curing salts.
Any potatoes should work, but these small white potatoes are delicious. After making broth, make mashed potatoes, or roast them.
Hold the water at a very gentle simmer for 30 minutes.
Keep skimming the broth throughout the cooking process. You want to remove all of the scum and fat.
Strain the broth. The bacon tends to be a little flavorless at this point, but the potatoes have picked up some of that bacon flavor and can be eaten as-is.