In small bowl, mix buckwheat honey with 1/2 cup/125 mL warm water; sprinkle yeast over top (if using Whey Sour Rye Starter, increase yeast to 1-1/2 tsp/7 mL). Let mixture stand until foamy.

Meanwhile, in large bowl, stir together starter, 1/2 cup/125 mL room temperature water and salt. In another bowl, whisk together rye and whole wheat flours. Stir yeast mixture into starter mixture. Sprinkle in half of the flour mixture; with wooden spoon, stir vigorously until smooth. Stir in remaining flour until sticky dough forms. Turn out onto lightly floured work surface; knead, adding additional rye flour if necessary, until smooth but somewhat sticky dough forms, 5 to 10 minutes. Shape into disc; sprinkle with walnuts and knead into dough. Shape into ball. Place in greased bowl, turning to coat; sprinkle lightly with rye flour. Cover and let rise in warm place until doubled, 2 to 3 hours.

Lightly punch down dough; knead lightly. Shape into ball, pulling down edges and pressing together in centre of bottom. Sprinkle pizza peel or rimless baking sheet generously with rye flour; place dough on peel or pan. Sprinkle top with rye flour. Cover with tea towel; let rise in warm place until almost doubled, about 1-1/2 hours.

With razor or very sharp knife, cut X, about 1/3 inch/8 mm deep, into top and two-thirds down side. Bake on preheated baking stone or on baking sheet in centre of 425F/220C convection oven or 450F/230C conventional oven for 10 minutes, sprinkling oven walls and bottom (avoid oven light) with about 1/3 cup/75 mL water once at beginning and once 3 minutes into baking.

Reduce heat to 350F/180C in convection oven or 375F/190C in conventional oven; bake until outside is crusty and bread sounds hollow when bottom is tapped, 35 to 45 minutes. Let cool completely on rack before slicing.

Makes about 12 slices.