Without looking specifically at your board, the HeapBlockDevice is a class made to prototype with BlockDevices without having any dedicated hardware, as long as your microcontroller has enough RAM available. Downside is that upon Reset or Power Cycling, all your data will be lost. Take a look at HeapBlockDevice - API references and tutorials | Mbed OS 6 Documentation for the documentation. I’ve had it working with the example provided by Mbed.
Also, you might want to check out STM32H743ZI2 QSPI Support, where you can find the kind of modifications you need to apply to your target to have it working with QSPI.
Hope it helps !