Fish in a Light Mustard Sauce

Ingredients (Serves 2)

1 lb fish ( 4-5 steaks,I used tilapia)
1 tsp virgin mustard oil
1/4 tsp turmeric powder
1 tsp oil

For the Sauce

1+1/4 teaspoon yellow mustard seeds
1/4 tsp nigella seeds
3 garlic cloves,minced
1/2 tsp turmeric powder
1/2 tsp virgin mustard oil
1/4 tsp red chili powder
2 tbsp water
3 tbsp virgin mustard oil
1/2 tsp fenugreek seeds
1 cup fresh tomato puree (just take 3 -4 medium tomatoes and process them to smooth in your food processor)
2 green chillies, slit open (adjust to tolerance)
1 tbsp finely chopped cilantro
Salt to taste
1/8 tsp sugar
1 - 1.5 cup water (depending on the sauce consistency you desire)

Notes 

The sauce from this recipe has a pronounced mustard taste and a bitter end note. We like it this way. 
However, you can reduce the mustard quantity to not less than 3/4 tsp for a mild flavor.

I used fresh roma tomatoes in this recipe so didn't feel the need to add any souring agent (like lemon juice or amchoor /dry mango powder) 
you could add them at the end of cooking if you feel that the sauce needs some tart.
Mustard oil is traditionally used in for the curry and it enhances the taste of the sauce. 
I cannot suggest a substitute but you can use any neutral oil.The curry will taste different though. 

Method

Clean the fish. Pat dry with a paper towel and rub with 1 tsp mustard oil and turmeric. Set in the refrigerator for 2 hours.

Grind mustard and nigella seeds to a powder (not very fine) using your coffee grinder. 
In a small bowl, mix with garlic, turmeric , 1/2 tsp mustard oil and red chili powder. Add 2 tbsp water to make a paste. 
Refrigerate this paste along with fish for 2 hours. (do not skip this refrigeration part)

After 2 hours, take out the fish and mustard paste from the refrigerator and set on the kitchen counter.

In a wide, non stick pan, heat up the 1 tsp oil on medium. 
Layer the marinated fish steaks on the pan and sear both the sides of the steak.
About 2 minutes on each side (this time will depend on the variety and thickness of your fish). Put the stove off and let sit.

Meanwhile, in a heavy bottomed pan/kadhai, pour the 3 tbsp mustard oil and heat it on medium till you see ripples on the surface.
Reduce the heat to low, wait for a minute and temper the oil with fenugreek seeds. 
Sauté for 20 seconds till you smell the aroma. 
Next, reduce heat to low and very carefully add the ground mustard paste. 
While adding to hot oil, the paste will splutter a lot so be careful.
Cook the paste on low heat for 2-3 minutes till you see oil starting to separate on sides of the pan. 
Add the salt, tomatoes puree and mix well.
Let cook on low heat,or about 10-12 minutes, stirring in between so that it does not stick to the bottom of the pan. 
Cook the paste till you see oil separating on the sides of the pan as well as it changing color to deep reddish-yellow. 
Make sure that all the water has evaporated and the masala is thick and shiny. Again, as the tomatoes cook,they splutters a lot, be careful.
Once the paste has cooked, add the water, green chillies, cilantro,check the salt and bring to a boil on medium low heat. 
Once boiling,add the pan fried fish along with the drippings in the pan if any. Also add the sugar. 
Cover and let cook for about 5-8 minutes. (this time will depend on the variety and thickness of your fish, cook till the fish is done)

Remove from heat, and let sit for at least an hour before serving. Serve warm with steamed rice. You could squirt some lemon juice at the end if you like.