The secure erase function still exists in El Capitan but not in Disk Utility. If (& ONLY if!) you are comfortable using Terminal, you can use the command line tool "diskutil" & its secureErase verb to securely erase either entire disks (very dangerous if you make a mistake!) or with the freespace option to securely erase just the free space on a volume (not very dangerous, relatively speaking, but you must remember to use the freespace option).
For the details see for example this MacIssues article or this MacWorld one. The man (manual) page for diskutil has been updated for 10.11, which you can access by typing "man diskutil" (without the quotes) in Terminal. To save you the trouble, I pasted the relevant part below:
secureErase [freespace] leveldevice
Erase, using a secure method, either a whole-disk (including
any and all partitions), or, only the free space (not in use
for files) on a currently-mounted volume. Erasing a whole-
disk will leave it useless until it is partitioned again.
Erasing freespace on a volume will leave it exactly as it was
from an end-user perspective, with the exception that it will
not be possible to recover deleted files or data using utility
software. If you need to erase all contents of a partition
but not its hosting whole-disk, use the zeroDisk or randomDisk
verbs. Ownership of the affected disk is required.
Level should be one of the following:
o 0 - Single-pass zero-fill erase.
o 1 - Single-pass random-fill erase.
o 2 - US DoD 7-pass secure erase.
o 3 - Gutmann algorithm 35-pass secure erase.
o 4 - US DoE algorithm 3-pass secure erase.
If you have any doubts at all about your Terminal skills, you would be better off using a utility, as you mentioned.
