Starter: In bowl, sprinkle yeast over 2/3 cup/150 mL warm water. Let stand until frothy, about 10 minutes. Stir in rye flour until smooth; scrape down side of bowl. Cover with plastic wrap; let stand at room temperature overnight. (Make-ahead: Let stand for up to 36 hours.)

Sponge: Stir 3/4 cup/175 mL and 2 tbsp/30 mL warm water into Starter. Stir in rye flour and bread flour until smooth. With wooden spoon, beat in only 1 direction for 100 strokes to set up gluten; scrape down side of bowl. Cover; let stand at room temperature overnight. (Make-ahead: Let stand for up to 18 hours.)

Dough: Scrape Sponge into large bowl. With wooden spoon, stir in 1/3 cup/75 mL warm water and salt; stir in 1 1/3 cups/325 mL of the rye flour and bread flour to make a heavy, sticky dough. Work in the remaining rye flour, adding heaping 1 tbsp/15 mL at a time, until shaggy but still sticky. Turn out onto floured work surface.

With well-floured hands and pastry scraper, knead dough, adding additional rye flour if dough is wet, until soft, elastic and no longer sticky, 5 to 7 minutes. (Or transfer shaggy dough to stand mixer and knead with dough hook on medium speed.) Form into ball. Place in greased bowl, turning to grease all over. Cover with plastic wrap; let rest in draft-free place at warm room temperature until doubled in bulk, about 1-1/2 hours.

Gently punch down dough. Return to floured work surface; knead lightly for a few turns. Cover with tea towel; let rest for a few minutes. Divide in half. If desired, press 1 half into disc and sprinkle with raisins; knead lightly until raisins are incorporated. Form each half into ball with taut surface by pulling sides down and folding into bottom at centre. Place on cornmeal-dusted bread peel if using bread stone, or on baking sheet. Cover with tea towel; let rise in draft-free place at room temperature until doubled in bulk, about 1 hour. If using bread stone, preheat for 30 minutes.

With razor or sharp serrated knife, cut six 1/4-inch/5 mm deep cuts from centre to side of each loaf, stopping about 1-1/2 inches/4 cm from bottom edge.

Bake in 375°F/190°C convection or 400°F/200°C conventional oven, spraying sides of oven with water (avoiding lightbulb) once just after placing bread in oven and again after first 3 minutes of baking, until tops are well crusted, interior temperature reads 205°F/96°C or loaves sound hollow when tapped on bottoms, about 45 minutes. Let cool on rack.

Makes 2 loaves of 8 to 10 slices each.