Circuit to "zoom in" on mV fluctuations of a DC signal?

Capacitors block DC and pass AC.

You can use a series capacitor into an opamp with whatever gain you need.

Even better might be a simple RC high-pass filter...One capacitor (series) and one resistor (to ground) in front of your amplifier.

Like this:

^{simulate this circuit – Schematic created using CircuitLab}

R2 and R3 set your gain. C1 and R1 set your low frequency cut-off. The formula you use to find the cutoff is:

$$F\text{(Hz)} = \frac{1}{2 \pi R C}$$

Here's something inspired by the first 2 answers. Make a 10-second low pass filter of the input signal and feed that into an op-amp's non-inverting input (+). Then take a 1-second high pass filter of the same input signal, and feed that into the inverting (-) input of the same op-amp.

Fluctuations get subtracted from the average and amplified a lot. If it's too much amplification, a resistor in series with C2 will lower the gain. This also inverts the fluctuation signals. If you want them non-inverted, follow this with a gain of -1 inverting stage.

^{simulate this circuit – Schematic created using CircuitLab}

Digital designer here so I'm not certain, but...

The other answers assume high-frequency fluctuations. Instead you want to subtract the 0.2 V and amplify that. You can use a summing amplifier to subtract the offset, if you've got positive and negative supply voltages. I think you can also use an inverting configuration where the non-inverting input is at 0.2V instead of ground.