Peel and roughly chop the beetroot. Place in a small saucepan with 2 tablespoons of water. Bring to a simmer, cover, and cook until very tender, about 15-20 minutes. Drain any excess water, then transfer the cooked beetroot to a food processor or blender and process until completely smooth. Set aside to cool.

In a large mixing bowl, combine the warm water, yeast, and sugar. Let it sit for 5-10 minutes until foamy.

Add the salt, olive oil, and beetroot puree to the yeast mixture and stir to combine.

Gradually add the flour, 1 cup at a time, mixing until a shaggy dough forms. Turn the dough out onto a lightly floured surface and knead for 8-10 minutes until smooth and elastic. If using a stand mixer, knead with the dough hook on medium speed for 6-8 minutes.

Lightly grease a clean large bowl with olive oil. Place the dough in the bowl, turning once to coat. Cover with plastic wrap or a damp cloth and let rise in a warm place for 60-90 minutes, or until doubled in size.

While the dough is rising, heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the minced garlic and sauté for 1 minute until fragrant.

Add the fresh spinach to the skillet in batches if necessary, stirring until it wilts completely. Cook for 2-3 minutes until most of the moisture has evaporated. Remove from heat.

Transfer the cooked spinach to a fine-mesh sieve and press out as much excess liquid as possible. Let cool slightly, then finely chop the spinach.

In a small bowl, combine the chopped spinach, crumbled feta cheese (if using), nutmeg, salt, and black pepper. Mix well and set aside.

Once the dough has doubled, gently punch it down and transfer it to a lightly floured surface.

Roll the dough into a large rectangle, approximately 12x18 inches (30x45 cm).

Spread the spinach filling evenly over the entire surface of the dough, leaving a 1/2-inch border along one long edge.

Starting from the long edge opposite the border, tightly roll up the dough into a log.

Place the seam-side down. Using a sharp knife or bench scraper, cut the log lengthwise down the middle, leaving about 1 inch (2.5 cm) uncut at one end.

Twist the two halves together, keeping the cut sides facing up to expose the filling. Pinch the ends together to seal, forming a braid.

Carefully transfer the braided dough to a baking sheet lined with parchment paper.

Cover the braided bread loosely with plastic wrap or a clean kitchen towel and let it rise in a warm place for another 30-45 minutes, or until visibly puffy.

Preheat oven to 375°F (190°C) during the last 15 minutes of the second rise.

In a small bowl, whisk together the egg and 1 tablespoon of water for the egg wash. Gently brush the entire surface of the bread braid with the egg wash. Sprinkle with sesame seeds if desired.

Bake for 30-35 minutes, or until the bread is golden brown and sounds hollow when tapped on the bottom. If the top browns too quickly, loosely tent with foil.

Remove from the oven and let cool on a wire rack for at least 15-20 minutes before slicing and serving.