In a large bowl, mix the spelt flour, bread flour, and water together by hand just until no dry flour remains. Cover and rest (autolyse) for 30 minutes.

Add the starter, salt, honey, and softened butter. Mix gently just until fully incorporated - a few minutes by hand is plenty. Avoid kneading hard or long; spelt's gluten is fragile and breaks down with too much handling.

Cover and bulk ferment at room temperature for 4–5 hours, giving the dough two gentle folds (fold each side in toward the center) in the first 2 hours, then leaving it undisturbed. It should look puffy and about 40–50% risen, not doubled.

Turn the dough out onto a lightly floured surface and shape gently into a log the length of your loaf pan. Place seam-side down in a greased 9x5-inch loaf pan.

Cover and refrigerate overnight (8–12 hours).

The next day, let the pan sit at room temperature for 45–60 minutes while the oven preheats to 375°F (190°C). The dough should crest just above the rim of the pan before it goes in.

Score the top lightly with a single lengthwise slash and bake for 35–40 minutes, tenting loosely with foil after 20 minutes if the top is browning too quickly.

The loaf is done when it sounds hollow when tapped and reads 195–200°F (90–93°C) in the center.

Turn out onto a wire rack and cool completely, at least 1 hour, before slicing.