Before performing a flash operation on your DUT you often want to put the DUT into a halted reset state.
 from lager import DUT

 mcu0 = DUT(mcu=0)

 # Your DUT my be a multi-mcu architecture
 # in which case you'll need more than 1 debug probe.
 mcu1 = DUT(mcu=1)

 # Halt your device
Halt DUT
~  lager debug reset --halt --mcu 0 --dut 1
~  lager debug reset --halt --mcu 1 --dut 1

You can also perform a reset and start operation by setting the halt flag to False.
 # Reset and start
Reset and Start DUT
~  lager debug reset --mcu 0 --dut 1