I've been hunting for the same thing since I moved to town. You can drive 75-90 minutes and really work hard for fish that'll average 5-8 inches, with the very occasional trout 12 inches or over. Or you can go an extra half hour/45 minutes to the Deschutes, Santiam or McKenzie, which are both stacked fisheries that are enough of a challenge to make you better every time you go.
Personally, I've yet to find a consistent spot within an hour/hour and a half of Portland proper. I've spent a lot of time bushwacking, trying to find my way around private property, and trying to find productive holes -- which equates to a lot less time with your flies in the water, figuring out which techniques work and which hatches to match. It's frustrating because it feels like there should be a gem or two closer to home, but the big rivers proximal to Portland are dominated by anadromous fish and the creeks are either stressed by the growth of human population or have super steep gradients that make it hard for trout to thrive.