Hydraulic energy recovery for high-pressure, low flow (non-steady state) conditions

This looks like an application for a hydraulic motor, these are typically camring, external lobe or internal lobe/geroller. Since (some) hydraulic machines work in short increments too, this should be doable.

You should hunt around for one the works with your silicon oil.

You should try to even out the flow as much as possible, since this will allow you to go for a smaller motor, smaller generator etc. Find out the longest time you have for unloading your system, and size the equipment for that.

Also i really doubt the 1500 bar. Are you sure you don't mean 1500 psi?

My perspective -

1. Phil's point should be considered first. The amount of energy available to recover is probably small relative to the cost of energy recovery equipment. However there are special cases with compressible vessels where energy storage may be significant. I'll answer the question anyway assuming you did all the homework on energy storage.
2. Recover energy @ 1500 bar. There are only a handful of manufactures for rotating hydraulic equipment anywhere near that pressure. Here is one I can recommend from having installed their equipment before: https://www.dynexhydraulics.com/product/pf4200-series-pumps/ The 4200 series lists a maximum pressure of 1380 bar. However Dynex often designs for custom applications with extreme pressures or temperatures (down to -50C). Also the 4200 series is listed as a pump but you can ask them about converting the unit for motoring operation.
3. Recover energy @ lower pressure. You could use a simple piston / cylinder to transform pressures. Take a cylinder with 3:1 area ratio. Input flow of silicone @ 1500 bar to the end with smaller surface area. The output flow will be @ 500 bar. Then you have many more options for hydraulic motors to recover energy. You would need a small secondary system to reset piston / cylinder position after each cycle.
4. One of the comments suggested expanding your 1500 bar fluid into a tank containing inert gas. This is certainly possible with significant energy dissipation as the comment noted. If you wanted to go this direction, here is one option for equipment - https://accumulators.com/piston-accumulators/available-models/#1496162639880-4e25dcdc-014b. Again these are rated up to 1380 bar but they should be able to get comfortably above that range with custom accumulators.
5. Side note - most hydraulic motor types are not worth checking for your application. Common varieties based on vane / gear / geroller / lobe / etc are rarely usable above 300 bar. Piston-type motors are the only option worth spending time on.