This bread will have 600g flour and 400g water altogether -- a 66% bread. 50g of water comes from the levain; the other 350g is what you've added. Similarly, the flours are made up of 100g that the levain contributes, and 500g that you've added on top of that.
Dissolve the starter in the water; you should get a milky, slightly viscous liquid.
Add bread flour, rye flour, high-gluten flour, rolled oats, and salt. Stir until it is a shaggy ball. Cover the bowl with a damp towel and let it sit for 30 minutes. (This is the autolyze.)
Knead the dough either by hand until it is well kneaded; or in a stand mixer with a dough hook for about 6 minutes on 3-speed.
When the dough has come together, add the currants and hazelnuts. Either knead them in by hand, or in the stand mixer on slow speed for about a minute.
Let the dough rise for its first fermentation in a dough bucket or a covered bowl. I let it rise at room temperature for up to 12 hours, allowing the flavors to develop. You can speed it up with a light-bulb warmed oven or placing the dough bowl in a larger bowl of warm water.
When the dough has at least doubled in volume (I've gotten it to triple), remove it from the bucket and form it into a loaf. Typically I use a boule shape, which is essentially a round loaf about the size of a soccer ball, and bake it directly on a stone. If you prefer to use a bread pan, that would work as well.
Let the bread rise for its final fermentation for around 2 hours.
Bake the bread in an oven preheated to 450F. If you have no baking stone but still want a free-form bread, you can use a preheated baking sheet.
After 35 minutes check the bread. An instant-read thermometer should tell you it is at 195-200F. If it's not, let the bread bake for another 5-10 minutes.
Let it cool. Then hide it where only you can find it. Else it will disappear miraculously.