If your problem is not solved by disabling the secure boot feature then you should have a look at the following link:
You may want to have a look at the section on the removable media path.
The section regarding the removable media path may be your problem, but I have no specifi knowledge if you Dell system has that problem.
If you think this may be the problem you can solve it manually to fix your installation using a bootable Debian usb stick and manually modify you EFI system Partition (ESP) as follows. Debian default UEFI boot code on ESP is /EFI/debian/grubx64.efi mounted on /boot/efi. Mount the ESP and create a new (as root) directory /EFI/boot and you copy /EFI/debian/grubx64.efi to /EFI/boot/bootx64.efi.
After this you should be able to boot. Keep in mind that you probably still have to do the grub install fix mentioned in the wiki to make sure the bootx64.efi is updated with the rest of the system.