Whisk the yeast, honey and 2½ cups (600 ml) lukewarm water in a medium bowl. Leave for 5 minutes until foamy or creamy. If nothing happens, your yeast may be inactive.
Add the flour and salt. Mix with a spatula until a shaggy dough forms and no dry spots remain.
Pour 4 Tbsp (60 ml) olive oil into a large bowl that fits in your fridge. Transfer the dough to the bowl and turn to coat. Cover and chill until doubled, at least 8 hours and up to 24 hours. In a rush? Let it rise at room temperature until doubled, 3–4 hours.
Generously butter a 33×23 cm baking pan for a thicker focaccia, or a 46×33 cm rimmed sheet pan for a thinner, crisper focaccia. Pour 1 Tbsp (15 ml) olive oil into the centre of the pan. Keeping the dough in the bowl, use two forks to lift the edges up and into the centre, turning the bowl a quarter turn and repeating 3–4 times to deflate and form a rough ball. Transfer to the prepared pan, pour over any oil left in the bowl and turn the dough to coat. Prove, uncovered, in a warm spot until doubled, 1½–4 hours.
Heat the oven to 230°C. The dough is ready when a finger poke springs back slowly, leaving a small indentation. Lightly oil your hands. If using a sheet pan, gently stretch the dough to fill. Dimple all over with your fingers, pressing right to the bottom. Drizzle with the remaining 1 Tbsp (15 ml) olive oil, scatter with flaky sea salt and rosemary (if using). Bake until puffed and deep golden, 20–30 minutes.
Just before serving, melt the butter in a small saucepan over medium heat. Take off the heat, grate in 2–4 garlic cloves, then return to the heat and cook, stirring, until just lightly toasted, 30–45 seconds. For a sharper garlic hit, grate the garlic into the hot butter off the heat and use immediately.
Brush the garlic butter all over the focaccia. Slice into squares or rectangles.
Make ahead: Focaccia is best the day it’s baked, but freezes well. Slice, freeze in an airtight container and reheat on a tray at 150°C.