Stir together the yeast and water in a small bowl. Stir in the spelt flour until mixture is fully combined.
Cover the bowl tightly with plastic wrap and let rest at room temperature for 12 to 24 hours. The resulting mixture should show a few bubbles and have a paste-like consistency.
To prepare the focaccia dough:
Whisk together the water and olive oil, and then add the active dry yeast.
In a mixing bowl, mix the rosemary, flours, and salt until combined, then stir in the water and yeast mixture.
Cover the bowl with a towel and let rest for 20 to 30 minutes. This step is essential to getting a great whole grain loaf.
Knead the dough for about 5 minutes, adding all-purpose flour as needed, until the gluten is formed and the dough is slightly springy to the touch. You want the dough to stay fairly moist - there will be some sticking, but not so much that the dough can't be handled.
Find a bowl large enough to hold twice the current volume of the dough, and grease it liberally with olive oil. Transfer the dough to the bowl and cover it with a warm, damp towel. Let rise in a warm place for 90-120 minutes, or until approximately doubled in size.
Fold two opposite edges of the dough lengthwise into the center, and then fold the other two edges into the center as well. Flip over the entire thing and put it back into the greased bowl. Cover it again, and let it rise for 30-60 more minutes, or it has gotten up to 50% larger again (the dough may have deflated when folded). It should feel bouncy/springy to the touch.
Divide the dough in half. Continue to the baking step, or deflate and store dough for up to 3 days in the fridge, or much longer in the freezer.
To prepare and bake the rosemary and meyer lemon focaccia:
Grease a cake pan or square pan with 2 tablespoons of olive oil, distributing it evenly around the bottom of the dish and a little bit up the sides.
Gently stretch the dough into a circular shape and place it into the oiled dish.
Let the dough rise, covered, in a warm place for about 30 minutes or until it has grown to start to fill out the baking dish.
Preheat the oven to 450 degrees Fahrenheit. Use a sharp knife or toothpick to poke holes in the top of the dough. Brush the top liberally with olive oil, then sprinkle with the chopped rosemary leaves. Layer the lemon slices sparsely on top (they should not overlap or cover the whole surface).
Brush the top, especially atop the lemon slices, with more olive oil.
Sprinkle the meyer lemon slices with sugar, and sprinkle the entire thing with coarse or regular salt.
Bake at 450 for 25 to 40 minutes, until the top is significantly browned and feels pretty firm to the touch. The areas directly under the lemon slices might not feel quite as firm.
Brush with another light coating of olive oil and transfer to the bottom rack of the oven to bake for 5 to 10 more minutes.
Remove from the oven and let cool at least 15 minutes before slicing.