Underproofed bread and how to fix it

Hi, I haven't made bread is ages, and when I did it never really turned out well. I somehow managed to get that oven spring in this one. The sourdough flavour is a little strong, but not too bad. However, I think it's a little underproofed. 

I used this recipe

• 500g bread flour
• 300g water (plus extra if needed)
• 10g salt
• 100g mature starter (suggested starter build 1:5:5 for 12 + hours overnight. 

Any tips? I put it in the fridge around 6pm for overnight and cooked at 9am at 450 covered for 20 min and 425 for 35 min. 

Thanks.