Bit of a change of plans for both the radishes and the mizuna. And a bit of an experiment too as neither ingredient is really known for its use in this sort of thing.
I started by frying up a pile of thinly-sliced radishes in copious butter and olive oil (since I knew the cooking fat would be the basis of the pasta sauce). Ideally, this should be over medium heat as the radishes go from raw to burnt rather quickly, but my stove's large burner only does high and off.
I overcooked at the time so here's a smaller batch I cooked up later at the level of doneness you're looking for: browned around the edges, some red left and just turning golden in the center. A little longer in the pan and they'll crisp up, but these are still soft. The radishes lose their bite early in the cooking process and turn sweet. The browning dims the sweetness and adds a toasty savoriness. At this stage, the flavor is not far from sweet potato chips (if you sprinkle on a bit of salt as they come out of the pan). It was hard to stop snacking on them, to tell the truth.
Sweet potato chips are also not known as a pasta topping so some additional ingredients are necessary. I added a few chopped anchovies for umami, but serano ham or even soy sauce would be good choices too if you wanted to take this in a different direction.
When the anchovies had dissolved and the pasta was done, I added the pasta to the pan, topped with the mizuna (which I had cleaned and removed the stemmier bits from. This required more attention than I expected as there were some rotty gunk mixed in that needed to be washed off. That's why I over-cooked my radishes. That's why you've got to have your mis en place all en place before you start.), removed the pan from the heat and tossed until the mizuna wilted a little.
Finish with a light drizzle of white wine vinegar (or a squeeze of lemon) to cut the fat and that's it. I never said it was a complicated recipe, just an interesting experiment. A successful one too, I'd say.
The radishes are taking the place of a more traditional toasted bread crumbs and the mizuna the place of a more traditional green (I considered the radish tops first, but their good to icky ratio was too low and would have taken too much time to deal with with the radishes already on the fire). Otherwise, a pretty standard Italian preparation and a pretty good one too.
My one reservation is that the radishes lost their crunch pretty quickly. I'd add some pinenuts next time. Or maybe capers. That would be nice too.