Oat flour adds a nice nutty flavour to these muffins. If you don’t have oat flour you can make some by whirling rolled oats around in a food processor until a fine powder is formed. All purpose flour can be subbed in for the whole wheat pastry flour if you don’t have that handy. The rhubarb from my garden was quite spindly; I used six stalks to come up with one cup of diced rhubarb. If your rhubarb is more robust, be sure to dice it into little bits. About 1 cm square pieces is what you’re after.