FOR THE TOPPING: Mix together lemon and sugar in a small bowl, until combined. Set aside.
Heat oven to 425 degrees. Bring 1 cup blueberries and 1 teaspoon sugar to simmer in small saucepan over medium heat.
Cook, stirring frequently and mashing berries with spoon/spatula several times
until berries breaks down and mixture is thickened and reduced to ¼ cup. Takes around 5-8 minutes. Switch the flame and cool to room temperature, 10 to 15 minutes.
In a large bowl, whisk flour, baking powder, and salt together.
Whisk remaining 1- 1/8 cups sugar and eggs together in another bowl
until thick and homogeneous, about 45 seconds.
Next, slowly whisk in (i use cultured butter) butter and oil until combined.
Add the buttermilk and vanilla until combined.
Slowly fold egg mixture
and then remaining cup blueberries into flour mixture until just moistened.
Batter will be very lumpy with few spots of dry flour. Don't OVERMIX.
Spray the muffin tin with some oil. Using an ice cream scoop or large spoon to divide batter equally among prepared muffin cups where the batter should completely fill cups and mound slightly.
Next drop a teaspoon of cooked berry mixture into center of each mound of batter.
Press it down to the middle of the batter.
Using chopstick or skewer, gently swirl berry filling into batter using figure-eight motion so that the jam is incorporated throughout the batter.
Sprinkle lemon sugar evenly over muffins. Since we do not prefer anything too sweet, I did not use the whole mixture - just about 2 tbsp or so.
Bake until muffin tops are golden and just firm, 17 to 19 minutes, rotating muffin tin from front to back halfway through baking time.
Cool muffins in muffin tin for 5 minutes,