I have a new gas cooktop and the lowest setting on the smallest burner still produces afast rolling boil rather that a very low simmer. How can I reduce the heat?
1. Simmer/Burner Plate
You can use a simmer plate or something to diffuse the heat. For example:
From the first link, a suggestion is to use a cast iron skillet as a heat diffuser. That should work in a pinch.
2. Adjust the Flame
Alternatively, some stoves allow you to adjust the flame level. I'm not a technician, so please do this at your own risk, read the manual, etc....
Since this is a new gas range, I'd suggest that you have the supplier confirm that the correct size orifices are installed. Generally speaking, most gas ranges come with two or more sets of orifices: Propane and Natural gas. Not only do those have different BTU ratings per unit of gas, they also normally run different pressures. In North America, natural gas is usually at 7" water column pressure, Propane runs at 11" water column pressure. Propane also contains more energy than Natural gas, which exacerbates the problem if you are running from Propane with Natural Gas orifices installed.
This is a common problem for single cooks who cook in small portions. A burner produces a constant energy output, which will bring different amounts of food to different end temperatures.
You can consider cooking a larger batch of food at once. This will give you a simmer and lots of tasty leftovers. Many liquid foods such as soups also freeze well.
Of course this is not the right solution for every case, but it's worth taking into account.
If you can't adjust or diffuse the heat enough, consider a separate electric cooktop. Single place electric burners are very common, inexpensive, and you may be able to adjust the heat output better. Look into induction cooktops when you do, you might be surprised at the control you have over your cooking with one of these.
I wanted to comment on another answer, but I don't have enough reputation.
On my stove, I am able to achieve a lower output towards the "off" position, than in the lowest output position.
The risk is that the flame may go out, especially if it is windy. This could essentially fill the room with unburned gas and be an explosion risk. But that is why most stoves have a thermocouple. If your stove has a thermocouple, it should be safe to use this method, since it would turn off the gas supply if there isn't any flame.