Context : Open clusters are recognised as excellent tracers of Galactic thin-disc properties .
At variance with intermediate-age and old open clusters , for which a significant number of studies is now available , clusters younger than \lesssim 150 Myr have been mostly overlooked in terms of their chemical composition until recently ( with few exceptions ) .
On the other hand , previous investigations seem to indicate an anomalous behaviour of young clusters , which includes ( but is not limited to ) slightly sub-solar iron ( Fe ) abundances and extreme , unexpectedly high barium ( Ba ) enhancements .

Aims : In a series of papers , we plan to expand our understanding of this topic and investigate whether these chemical peculiarities are instead related to abundance analysis techniques .

Methods : We present a new determination of the atmospheric parameters for 23 dwarf stars observed by the Gaia -ESO survey in five young open clusters ( \tau < 150 Myr ) and one star-forming region ( NGC 2264 ) .
We exploit a new method based on titanium ( Ti ) lines to derive the spectroscopic surface gravity , and most importantly , the microturbulence parameter .
A combination of Ti and Fe lines is used to obtain effective temperatures .
We also infer the abundances of Fe i , Fe ii , Ti i , Ti ii , Na i , Mg i , Al i , Si i , Ca i , Cr i , and Ni i .

Results : Our findings are in fair agreement with Gaia -ESO iDR5 results for effective temperatures and surface gravities , but suggest that for very young stars , the microturbulence parameter is over-estimated when Fe lines are employed .
This affects the derived chemical composition and causes the metal content of very young clusters to be under-estimated .

Conclusions : Our clusters display a metallicity [ Fe/H ] between +0.04 \pm 0.01 and +0.12 \pm 0.02 ; they are not more metal poor than the Sun .
Although based on a relatively small sample size , our explorative study suggests that we may not need to call for ad hoc explanations to reconcile the chemical composition of young open clusters with Galactic chemical evolution models .