Press Sauté. Melt the butter, then add the onion and carrots. Cook 3–4 minutes, until the onion is translucent. Add the garlic and cook 30 seconds more.
Sprinkle the flour over the vegetables and whisk constantly for 1–2 minutes. The mixture will look like wet sand.
Press Cancel. Add the broth a splash at a time, whisking smooth. Then scrape the bottom of the liner thoroughly with a wooden spoon until nothing is stuck. Do not skip this — it prevents the BURN notice.
Add the broccoli florets on top of the liquid along with the salt, pepper, nutmeg, and dry mustard. Do not stir.
Lid on, valve to Sealing. Cook on high pressure for 1 minute. It will take about 10 minutes to come to pressure.
Quick release the pressure, carefully, keeping hands and face away from the valve.
Press Cancel so no heat is running. Stir in the half-and-half.
Add the cheddar in 3–4 handfuls, stirring until each one melts completely before adding the next.
Taste, adjust salt and pepper, and serve hot with extra cheddar on top.