Line a 9x5 inch loaf pan with parchment paper, allowing some overhang on the sides for easy removal. Preheat your oven to 350°F so it reaches the proper temperature by the time you're ready to bake. This gives you time to prepare and mix your batter while the oven heats.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Whisk for about 30 seconds to combine and aerate the dry ingredients—this helps distribute the leavening agents evenly throughout the batter and prevents lumps that can affect the bread's texture.

In a separate large bowl, add the softened butter and olive oil together, then beat in the sugar until the mixture is light and fluffy, about 2-3 minutes—this incorporates air and creates a tender crumb. Add the room-temperature eggs one at a time, beating well after each addition. Stir in the sourdough discard, milk, lemon juice, lemon zest, vanilla, nutmeg, and ground nutmeg until well combined. The sourdough discard adds complexity and tang that complements the bright lemon and sweet blueberries beautifully.

Pour the wet mixture from Step 3 into the dry ingredients from Step 2, stirring until just combined—do not overmix, as this can develop gluten and create a tough, dense bread. The batter should look slightly lumpy. Toss the blueberries in a little flour (this coating helps prevent them from sinking to the bottom during baking), then gently fold them into the batter using a rubber spatula with just 5-6 folds to keep them intact.

Pour the batter from Step 4 into the prepared pan from Step 1 and smooth the top. In a small bowl, combine the cold butter cubes, flour, sugar, and salt for the streusel topping, rubbing the mixture between your fingers until it resembles coarse breadcrumbs. Sprinkle the streusel evenly over the batter—I love how this adds a subtle crunch and keeps the top from browning too quickly.

Place the pan in the preheated 350°F oven and bake for 60-70 minutes, until a toothpick inserted in the center comes out with just a few moist crumbs (not wet batter). The streusel should be golden brown. If the top is browning too quickly before the bread is done, tent it loosely with foil for the remaining baking time.

Let the bread cool in the pan for 20 minutes, then turn it out onto a wire rack and cool completely before glazing. In a small bowl, whisk together the sifted powdered sugar and lemon juice until smooth and pourable—sifting the powdered sugar prevents lumps that can make the glaze grainy. Drizzle the glaze over the cooled bread and let it set for a few minutes before serving.