Place barley in a jar and cover with water. Seal jar and let stand 24 hours.
Drain, saving the liquid, rinse and place back in the jar. Cover with a towel and let stand, turned upside down, overnight. Repeat the rinse / drain process three times more, saving the water each time.
Wash dried yellow split peas and place them in a medium sized bowl. Cover with water and leave overnight to soak.
Mix together the starter, flours, salt and 350 mL barley water (drained from step 2) until a thick wet dough forms. Leave in the fridge, covered, overnight. Let stand at room temperature 1 hour to warm up before proceeding.
Drain split peas and add to a food processor with the barley. Pulse to coarsely chop, set aside.
Knead rested dough for 10 minutes with the dough hook or 20 minutes by hand, adding flour as necessary to achieve a slightly sticky, but not wet, dough.
Knead in the chopped split pea mixture.
Form into a ball and place on a heavily floured tea towel.
Place towel-wrapped loaf in a basket, bowl or banneton and leave to rise 24 hours at cool room temperature (don’t try to shortcut this step - it’s a heavy loaf and needs a *long* rise).
Place a large baking tray or pizza stone in the oven and heat oven to 450F (preferably convection).
Turn loaf out onto a parchment lined rimless baking sheet or pizza peel and transfer to the hot pan or baking stone.
Score loaf with a sharp knife and place in the oven.
Bake for 10 minutes, then turn the oven to 425F (preferably convection) and bake another hour.