In the English Grammar Profile, C2 point 66 in the category of FUTURE is defined as:
INVERSION present simple with ‘only when’ (followed by ‘will’ and inverted subject) to refer to the future.
A search on Google News:
The Irish TimesA search in Ted corpus for:
only when * will {pr}
only when you really care will they leave their fears behind
This is an incredibly rare structure.
An iWeb search for:
only when _P _V
1 ONLY WHEN IT IS 3008
Then a check for ‘will’ in COCA results shows that finding ‘will’ is less than 10% and inversion does not exist.
1 2019 SPOK Fox_Baier
The bill states that officers will be able to use lethal force only when it is necessary, and if there are no other options.
74 1999 ACAD MusicEduc
Children will need more assistance, of course, but the teacher should provide help only when it is needed and stay out of the way when it is not.
2 ONLY WHEN THEY ARE 2080
3 ONLY WHEN YOU ARE 1978
4 ONLY WHEN YOU HAVE 1544
5 ONLY WHEN YOU NEED 841
6 ONLY WHEN IT ‘S 813
7 ONLY WHEN YOU ‘RE 647
8 ONLY WHEN WE ARE 603
9 ONLY WHEN THEY HAVE 493
10 ONLY WHEN WE HAVE 483
11 ONLY WHEN I ‘M 460
12 ONLY WHEN I WAS 445
13 ONLY WHEN YOU WANT 351
14 ONLY WHEN YOU GET 343
15 ONLY WHEN I AM 333
16 ONLY WHEN IT COMES 329
17 ONLY WHEN HE IS 328
18 ONLY WHEN I HAVE 304
19 ONLY WHEN IT WAS 291
20 ONLY WHEN THEY ‘RE 282
21 ONLY WHEN IT HAS 230
22 ONLY WHEN YOU CAN 227
23 ONLY WHEN THEY WERE 226
24 ONLY WHEN HE WAS 223
25 ONLY WHEN YOU USE 221
26 ONLY WHEN YOU FEEL 211
27 ONLY WHEN YOU KNOW 204
28 ONLY WHEN YOU DO 202
29 ONLY WHEN THEY NEED 176
30 ONLY WHEN YOU START 161
31 ONLY WHEN I STARTED 160
32 ONLY WHEN HE HAS 156
33 ONLY WHEN I GOT 145
34 ONLY WHEN I NEED 144
35 ONLY WHEN SHE IS 131
36 ONLY WHEN YOU ‘VE 125
37 ONLY WHEN I HAD 121
38 ONLY WHEN YOU SEE 119
39 ONLY WHEN THEY CAN 118
40 ONLY WHEN I FEEL 104
41 ONLY WHEN THEY FEEL 102
42 ONLY WHEN SHE WAS 102
43 ONLY WHEN THEY GET 101
44 ONLY WHEN YOU LOOK 99
45 ONLY WHEN I WANT 97
46 ONLY WHEN IT CAN 89