softly whipped cream, yogurt, or crème fraiche, or for serving
Position a rack in the center of the oven and preheat to 350ºF.
Rub a 10-inch round solid tart pan, or 9-inch cake pan or springform pan, with the butter and place on a rimmed baking sheet to catch any drips.
Trim away any leafy bits from the rhubarb. Slice the stalks diagonally 1-inch thick, then dice any less attractive pieces until you have ¾ cup diced rhubarb. Toss the sliced rhubarb with 3 tablespoons sugar and a squeeze of lemon juice, and toss the diced rhubarb in a separate bowl with 1 tablespoon sugar and squeeze of lemon juice. Set these aside while you prepare the cake batter or up to 30 minutes.
In a large bowl, rub the lemon zest into ½ cup of the sugar until moistened and clumpy. Whisk in the eggs, yogurt, and olive oil until combined.
Place a strainer over the bowl (if your bowl is tall enough, otherwise do this into another bowl) and sift in the almond, sweet rice, oat and millet flours along with the baking powder, baking soda and salt, adding in any bits that get caught in the sifter.
Stir the flour mixture into the egg mixture until smooth and homogenous. Drain the diced rhubarb and stir it in until combined. (The rhubarb juice makes a nice "soda" with fizzy water and ice!)
Scrape the batter into the prepared pan, and smooth into an even layer. Top with the sliced rhubarb in concentric circles if you like (or just drop them willy-nilly over the batter – that looks fun too!) Leave behind any rhubarb juice and add it to fizzy water for a little treat if you like!
Sprinkle all over with the remaining 2 tablespoons (20 g) sugar
Bake the cake until golden on top and a tester inserted near the center comes out clean, 40–50 minutes.
Remove the cake from the oven and let cool completely, 45 minutes, then remove the sides from the pan. Slide the cake onto a serving platter and cut into wedges to serve.
The cake is best within the first 2 days of baking. Store, covered, at room temperature the first day, then pop in the fridge to keep it fresh if storing longer.
Grain-Free Rhubarb Cake: Omit the sweet rice, oat, and millet flours and use ½ cup + 2 tablespoons (95 g) cassava flour and ¼ cup (30 g) tapioca flour.Dairy-Free Rhubarb Cake: Use plant yogurt instead of Greek yogurt. A rich coconut yogurt such as Culina, Coyo, or Cocojune would be good choices.No rhubarb? No problem! Swap it out for any seasonal fruit: berries, cherries, plums, stone fruit, ripe pears, and figs would all work well.