In a large bowl, combine the flour, sugar, and salt. Set aside.
Pour the warm water into a small dish and mix in the yeast. Let the mixture rest for about 5 minutes.
Beat the butter and egg until well-combined. Add in the yeast mixture and whisk to combine.
In small parts, mix in the flour mixture. Once the dough thickens, you may have to use a sturdy wooden spoon to work it until it turns elastic. Alternatively, you can use a stand mixer to make the process a bit easier.
Cover the bowl and let the dough rise for about an hour or until it's doubled in volume.
Grease a 9x13-inch baking pan and spread the dough evenly within it. Let the dough rest for 30 minutes.
Drizzle generously with olive oil. Using your fingertips, poke holes all over the dough to make shallow dents. Sprinkle the top with rosemary and flaky sea salt.
Bake the bread at 400°F for about 20 minutes or until the top is golden brown.
Remove the bread from the pan. Serve warm or cold and enjoy!