In a small saucepan over low heat, heat the half-and-half with the butter until the butter melts. Set aside and cool to lukewarm.
Zest the lemon, scraping only the yellow rind (you'll have 1 to 2 teaspoons of zest). Cut the lemon in half and squeeze the juice, straining out any seeds.
In a small bowl, whisk together lemon zest, 1 cup flour, yeast, salt and nutmeg (if using). Set aside.
In the bowl of a stand mixer with a paddle attachment, beat eggs with sugar and sour cream until smooth. With the mixer on low speed, gradually add lukewarm half-and-half mixture and lemon juice and mix well. Add flour mixture and mix until smooth. Beat in 3 additional cups of flour, 1 at a time, mixing well. The dough will be soft and quite sticky. Switch attachment to a dough hook and knead dough for 3 to 4 minutes, until smooth. It will remain slightly sticky.
(If mixing by hand, follow the mixing process through adding 3 cups flour. To knead, turn the dough out onto a floured surface. Use a dough scraper [also called a bench knife] to lift and fold the dough, stretching it as much as possible as you lift with the scraper. Flour as necessary using the remaining 1 cup flour, but use as little as possible. Stretch and fold for several minutes, until the dough becomes smoother and firmer.)