Was sertraline effective? If so, it might have been better to stay with it and add something for the insomnia. Small doses of immediate-release trazodone (Desyrel) has become the defacto treatment for SSRI induced insomnia (the slow release Oleptro version is a pretty good antidepressant, btw, with few of the typical antidepressant side-effects which really bug people such as their impact on the libido).
You could try increasing the mirtazapine. It is mostly just a powerful antihistamine so sedation is what it does best.
You could try also try the afore mentioned trazodone (Desyrel), but I suspect your doctor would freak at the mere thought because his drug interaction check will probably, incorrectly, highlight a potential for serotonin syndrome.
Unfortunately, much of the medical literature on the syndrome is wrong. Not my words, but that of arguably the leading authority on the subject, Dr Ken Gillman (he calls it serotonin toxicity).
To quote:
Neither drug is a serotonin reuptake inhibitor, or a serotonin releaser so serotonin syndrome is simply not possible.