It's quite complicated, but I'll try to explain. Note that I've inserted the word "that" to make the analysis clearer for you:

MAIN CLAUSE: "The only one that he was interested in spearing was Boris".

The subject is the noun phrase "The only one that he was interested in spearing".
The verb is "was".
The subject complement is "Boris".
The relative clause is "that he was interested in spearing".

SUBORDINATE RELATIVE CLAUSE: "That he was interested in spearing".

The relative word is "that" (it refers to "the only one")
The subject is "he".
The verb is: "was".
The subject complement is: "interested" in spearing ________".

Note that the gap notation ______ represents the grammatical position of the object of "spearing", i.e. "the only one".

I hope that makes sense!

PaulM