Pour oil into a 10-inch oven-proof dish and place in oven to heat.
In large bowl, whisk eggs well.
Add milk and vanilla to eggs and whisk until well blended.
Add flour, sugar, cinnamon and salt and mix until smooth.
Remove hot dish from oven.
Sprinkle rhubarb slices evenly on bottom of pan.
Pour batter over rhubarb and immediately return to oven.
Bake for 25 to 30 minutes until puffy and golden brown throughout with raised, darker brown edges. The puffing happens towards the end of baking, be patient.
Sprinkle with confectioner’s sugar and serve immediately.
Notes
I do not recommend using frozen rhubarb for this recipe, it releases too much moisture. If that's your only option, toss frozen rhubarb into your hot pan for 3-4 minutes to warm and evaporate extra moisture. Then add your batter and pop it in the oven. It may take a few more minutes to bake.