Personally adlib good hay, a small amount of hard feed, light mix or similar and NAF olivite. We keep natives and cobs and to be honest it doesn't do them any good pumping large amounts of hard feed into them unless they are working very hard.
If it was me I'd feed 2 scoops of happy hoof split between 2 feeds. Also try and lunge her and walk out in hand to help build muscle. Once she starts putting on weight reduce her feed to just one scoop in the evening.
SLOWLY, very slowly, using a feed balancer and low calorie roughage (inluding controlled grazing, especially if it is good grass) Laminitis Trust Approved feedstuffs in the quantities they advise and a very careful eye on her weight gain.
She may still be growing (you don't mention age) and nothing will fill out before it has stopped growing 'up' (wards).
Don't rush her maturing, and consider delaying the breaking in. Not all are ready at the conventional time and you risk damaging growing tissue by artifically speeding up this process.
Please take it steady - in every sense of the word. She is a baby (I presume) and you could be the undoing by going mad to get her 'shiney and full up'. It doesn't work that way with natives...
Agreed. My Welshy was 8 before he really matured and was able to work properly.
Personally I'd have her out as much as possible on grass. She won't develop muscle through feeding, that comes slowly through correct and sympathetic work.