Heat a saucepan over medium-high heat. Add the olive oil and onion. Cook the onion until translucent, about 5 minutes. Add the sardine, garlic, oregano, basil, sugar, salt, and pepper. Break up the sardine and cook for 5 minutes, mixing to not have anything stick to the bottom of the pan.

Crush the whole tomatoes by hand and add them along with the tomato sauce. Bring to a simmer and then lower the heat. Cover and let it cook for 1 ½ hours, until it reduces by ¼. Make sure to stir the sauce every ten minutes or so to avoid it sticking to the bottom of the pan. After the sauce has reduced, combine the cornstarch and water in a small bowl. Add to the saucepan and mix in well to help thicken the sauce. Remove from the heat and set aside.

Heat a medium frying pan with nonstick spray over high heat. Add the Italian sausage and cook, flipping to brown all sides, until the sausage is cooked through and reaches an internal temperature of 160°F, about 15 to 20 minutes. Transfer to a cutting board and let rest for 5 minutes.

Chop the sausages into bite-sized chunks and place in a medium bowl. Add the sauce and all the cubed cheese. Toss together until well mixed.

It is time to prepare the filling into individual portions. Using a standard sized muffin tin, place a rectangular piece of plastic wrap (about 5-inches by 4 inches, enough to cover the filling when placed) in a hole of the muffin tin. Fill with the sauce mixture to the top of the hole. Wrap the mixture with the plastic wrap. Repeat with the remaining sauce. You should end up with 12 portions. Place in the freezer and let set for about 2 hours. This will make it much easier to assemble.

While the sauce is setting, take the time to prepare the bao dough. In a bowl, mix the yeast, water, and oil. Allow this to rest for 5 minutes or until the yeast becomes active. In the bowl of a stand mixer, combine the flour, sugar, baking powder, garlic powder, onion powder, and ground fennel. Set the mixer to the lowest setting and slowly add the yeast mixture. Mix until it all comes together.

Transfer to a lightly floured surface and knead for 3 minutes. Place the dough in an oiled bowl and cover. Let it rest until it doubles in size, about 2 hours.

When you are ready to make the bao, place water in a pot with a steamer basket and bring to a boil. Punch the dough down and roll out into a long tube. Split into 12 equal pieces and form into balls.

Take a portion and roll out to about 6 inches wide. Avoid making it too thin. Remove one of the frozen sauce pieces from the plastic wrap and place in the center of the dough.

Pleat the buns until sealed. Place on top of a small piece of parchment paper and then place in a steam basket (do not place the steam basket on the pot until all are shaped). Repeat until all the bao are shaped. Make sure the bao aren’t touching.

Once the water is boiling, place the steamers on top of the pot and steam for 20 to 22 minutes, until the bao dough is cooked and the filling is cooked through.