You can use a single transistor to do the job.
Feed the hall sensor output through a potentiometer. Adjust it so the transistor just starts to turn on at the voltage you specified.
You said you want increasing output V with increasing hall sensor V. Therefore take your output at the emitter. Install a pull-down resistor to ground.
The above can be done with an NPN.
In case it doesn't work right, then you might make headway using a PNP instead. Recall that its bias must be referenced to positive. In that case connect one end of the potentiometer to the positive supply (not to ground).
You can also use a comparator if you wish. Its output is able to go to ground, unlike a lot of op amps. Use a pot across the supply rails. Feed the wiper terminal to a comparator input. Set the voltage where you want the comparator to change state. The output will need a pullup resistor so that it can give you a positive reading.