Insomuch means "to such an extent" or "to such a degree." You may have had a bad day, but at least it was a beautiful one, insomuch as the sun was shining and the sky was blue.
You'll always find the adverb insomuch preceding "as" or "that," and it can be used interchangeably with inasmuch. Both words started as phrases during the 13th century. You might tell your friends that your dad is retired, insomuch that he's not going in to his office, although he's still working from home. Or that your dog is well trained, insomuch as she comes when you call — if you happen to be holding a piece of cheese.